Mask processing using films with spatially selective birefringence reduction

ABSTRACT

Certain patternable reflective films are used as masks to make other patterned articles, and one or more initial masks can be used to pattern the patternable reflective films. An exemplary patternable reflective film has an absorption characteristic suitable to, upon exposure to a radiant beam, absorptively heat a portion of the film by an amount sufficient to change a first reflective characteristic to a different second reflective characteristic. The change from the first to the second reflective characteristic is attributable to a change in birefringence of one or more layers or materials of the patternable film. In a related article, a mask is attached to such a patternable reflective film. The mask may have opaque portions and light-transmissive portions. Further, the mask may have light-transmissive portions with structures such as focusing elements and/or prismatic elements.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/703,549, filed Dec. 11, 2012, now pending, which is a 371 ofInternational Application No. PCT/US2011/42368, filed Jun. 29, 2011,which claims benefit of U.S. Provisional Application No. 61/360129,filed Jun. 30, 2010, the disclosure of which is incorporated byreference in their entirety herein.

FIELD OF THE INVENTION

This invention relates generally to optical films, with particularapplication to optical films whose reflective characteristics can bespatially tailored by selective application of radiant energy tailoredto relax the birefringence of a constituent layer or material of theoptical film, and the invention also relates to associated articles,systems, and methods.

BACKGROUND

Spatially tailored optical films, sometimes referred to as “STOF” films,are described in several commonly assigned but currently unpublishedinternational and U.S. patent applications which are referenced at theend of the Detailed Description. In some cases, these films incorporateinternal structural features that provide the film with an initialreflective characteristic. For example, the internal structural featuresmay be one or more packets of microlayers that selectively reflect lightby constructive or destructive interference, or the internal structuralfeatures may be polymer materials that are separated into distinct firstand second phases in a blended layer to diffusely scatter light. Thefilms also have an absorptive characteristic that allows them to betreated with radiant energy at any desired locations or zones within auseable area of the film, the radiant treatment causing the initialreflective characteristic to change to a different second reflectivecharacteristic at the treated locations. The change occurs primarilybecause of a birefringence relaxation mechanism: the film absorbs anappropriate amount of the radiant energy because of the absorptivecharacteristic, the absorbed energy heats the film at localizedpositions, i.e., at the desired locations or zones, and the heat causesthe birefringence of one or more constituent layers or materials of thefilm in such desired locations or zones to relax, e.g, to become lessbirefringent or to become isotropic. In exemplary cases, the radiantenergy does not heat the film enough to substantially change or damage astructural integrity (e.g., layer structure or immiscible blendmorphology) of the film in the treated zones.

BRIEF SUMMARY

We have developed techniques of using reflective STOF films as patternedmasks in the manufacture of other patterned articles. The otherpatterned articles may include other STOF films, and/or they may includeother types of patternable articles. We have also developed techniquesof using conventional masks in the manufacture of STOF films. We havefurther developed articles in which a reflective STOF film is connectedto a mask in a layered arrangement. The connection between the STOF filmand the mask allows the STOF film to be patterned in a zone that is inspatial registration with a feature such as a transmissive area or anopaque area of the mask.

Thus, certain patternable reflective films are used as masks to makeother patterned articles, and one or more initial masks can be used topattern the patternable reflective films. An exemplary patternablereflective film has an absorption characteristic suitable to, uponexposure to a radiant beam, absorptively heat a portion of the film byan amount sufficient to change a first reflective characteristic to adifferent second reflective characteristic. The change from the first tothe second reflective characteristic is attributable to a change inbirefringence of one or more layers or materials of the patternablefilm. In a related article, a mask is attached to such a patternablereflective film. The mask may have opaque portions andlight-transmissive portions. Further, the mask may havelight-transmissive portions with structures such as cylindricalelements, focusing elements, and/or prismatic elements.

The present application therefore discloses, inter alia, methods ofmaking patterned films that include providing a first film having afirst reflective characteristic, and providing a second film having afirst detectable characteristic. The first film may also have a firstabsorption characteristic suitable to, upon exposure to a first radiantbeam, absorptively heat a portion of the first film by an amountsufficient to change the first reflective characteristic to a secondreflective characteristic by a change in birefringence. The firstdetectable characteristic of the second film may change to a differentsecond detectable characteristic by exposing the second film to a secondradiant beam. The methods may also include directing the first radiantbeam preferentially at a second zone rather than a first zone of thefirst film to change the first reflective characteristic to the secondreflective characteristic in the second zone so as to convert the firstfilm to a patterned mask. The methods may further use the patterned maskto pattern the second radiant beam, and direct the patterned secondradiant beam at the second film to change the first detectablecharacteristic to the second detectable characteristic at selectedportions of the second film.

The first film may include a first group of interior layers arranged toselectively reflect light by constructive or destructive interference toprovide the first reflective characteristic, and the change from thefirst reflective characteristic to the second reflective characteristicmay be substantially attributable to a change in birefringence of atleast some of the interior layers. The first film may instead include afirst blended layer that includes first and second polymer materialsseparated into distinct first and second phases, respectively, and thechange from the first reflective characteristic to the second reflectivecharacteristic may be substantially attributable to a change inbirefringence of at least one of the first and second polymer materials.In some cases, the first film may be or comprise a bi-level patternedreflective film.

The procedure of directing the first radiant beam preferentially at thesecond zone of the first film may include scanning the first light beamover portions of the first film that define the second zone. The firstreflective characteristic may reflect the second radiant beam more thanthe second reflective characteristic, and the selected portions of thesecond film may correspond to the second zone of the first film.Alternatively, the first reflective characteristic may reflect thesecond radiant beam less than the second reflective characteristic, andthe selected portions of the second film may correspond to portions ofthe first film other than the second zone.

The first and second radiant beams may include different first andsecond optical wavelengths, respectively. For example, the first opticalwavelength may be an infrared optical wavelength, and the second opticalwavelength may be less than 700 nm. In another example, the first andsecond optical wavelengths may be different infrared wavelengths, e.g.,808 nm and 1064 nm. The second film may have a second absorptioncharacteristic suitable to, upon exposure to the second radiant beam,absorptively heat a portion of the second film by an amount sufficientto change the first detectable characteristic to the second detectablecharacteristic. The second film may include a second group of interiorlayers arranged to selectively reflect light by constructive ordestructive interference to provide the first detectable characteristic,and the change from the first detectable characteristic to the seconddetectable characteristic may be substantially attributable to a changein birefringence of at least some of the interior layers. Alternativelyor in addition, the second film may include a second blended layer thatincludes third and fourth polymer materials separated into distinctthird and fourth phases, respectively, and the change from the firstdetectable characteristic to the second detectable characteristic may besubstantially attributable to a change in birefringence of at least oneof the third and fourth polymer materials.

The first or second reflective characteristic may have a reflectivityfor the second radiant beam, and/or at another wavelength of interest,of at least 90%, or at least 95%, or at least 99%.

The directing of the first radiant beam may provide the first film witha first pattern, and the method further include, after using thepatterned mask to pattern the second radiant beam, directing a thirdradiant beam at the first film to eliminate at least a portion of thefirst pattern in the first film. The first and second films may beconnected in a layered arrangement. The third radiant beam may betailored to render the first film substantially unpatterned, e.g., itmay eliminate substantially all of the first pattern in the first film,or it may eliminate only a portion of the first pattern so as to providea different second pattern in the first film. The second reflectivecharacteristic may be less reflective than, or more reflective than, thefirst reflective characteristic.

We also disclose methods of making patterned films, which may includeproviding a patterned mask and providing a first film. The first filmmay have a first reflective characteristic, as well as a firstabsorption characteristic suitable to, upon exposure to a first radiantbeam, absorptively heat a portion of the first film by an amountsufficient to change the first reflective characteristic to a secondreflective characteristic. The methods may further include using thepatterned mask to pattern the first radiant beam, and directing thepatterned first radiant beam at the first film to change the firstreflective characteristic to the second reflective characteristic atselected portions of the first film.

The first film may include a first group of interior layers arranged toselectively reflect light by constructive or destructive interference toprovide the first reflective characteristic, and the change from thefirst reflective characteristic to the second reflective characteristicmay be substantially attributable to a change in birefringence of atleast some of the interior layers. The first film may alternatively orin addition include a blended layer that includes first and secondpolymer materials separated into distinct first and second phases,respectively, and the change from the first reflective characteristic tothe second reflective characteristic may be substantially attributableto a change in birefringence of at least one of the first and secondpolymer materials. Substantially all of the selected portions of thefirst film may be changed from the first to the second reflectivecharacteristic at a same time.

We also disclose articles that may include a first film attached to amask in a layered arrangement. The first film may have a firstreflective characteristic, and may also have a first absorptioncharacteristic suitable to, upon exposure to a first radiant beam,absorptively heat a portion of the first film by an amount sufficient tochange the first reflective characteristic to a second reflectivecharacteristic.

The change from the first reflective characteristic to the secondreflective characteristic may be substantially attributable to a changein birefringence of at least a portion of the first film. The first filmmay include a first group of interior layers arranged to selectivelyreflect light by constructive or destructive interference to provide thefirst reflective characteristic. The first film may alternatively or inaddition include a blended layer that includes first and second polymermaterials separated into distinct first and second phases, respectively,and the first and second reflective characteristics may include firstand second diffusely reflective characteristics respectively.

The mask may have a useable area, and some portions of the useable areamay block the first radiant beam and other portions of the useable areamay transmit the first radiant beam. The mask may alternatively or inaddition include one or more structured surface features adapted topreferentially redirect the first radiant beam onto selected portions ofthe first film. The one or more structured surface features may includea cylindrical element, a focusing element, and/or a prismatic element.The one or more structured surface features may be adapted to cause theselected portions of the first film to be adequately heated by theamount sufficient to change the first reflective characteristic to thesecond reflective characteristic, and to cause other portions of thefirst film to not be adequately heated by the amount sufficient tochange the first reflective characteristic to the second reflectivecharacteristic.

Related methods, systems, and articles are also discussed.

These and other aspects of the present application will be apparent fromthe detailed description below. In no event, however, should the abovesummaries be construed as limitations on the claimed subject matter,which subject matter is defined solely by the attached claims, as may beamended during prosecution.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a schematic side or sectional view of components involved invarious steps of a process that uses a reflective STOF film as a maskfor patterning another article;

FIG. 1B is a schematic side or sectional view of components involved invarious steps of a process in which a mask is used to pattern areflective STOF film;

FIG. 2 is a perspective view of a roll of reflective STOF film that hasbeen internally patterned to provide different reflectivecharacteristics in different portions or zones of the film so as to formindicia;

FIG. 3A is a schematic side view of a portion of a multilayer opticalfilm;

FIG. 3B is a schematic perspective view of a portion of a blended layerof a diffuse optical film;

FIG. 4 is a schematic sectional view of a portion of the reflective STOFfilm of FIG. 1;

FIG. 5 is a schematic sectional view of a portion of a reflective STOFfilm with internal patterning;

FIGS. 5A-D are idealized plots showing each refractive index (nx, ny,nz) of two alternating microlayers of a microlayer packet, or of twodistinct polymer materials of a blended layer, for different stages ofmanufacture of various reflective STOF films;

FIG. 6 is a schematic diagram that summarizes various transformationsthat can be achieved using the techniques discussed herein forreflective STOF films;

FIG. 7 is a schematic side view of an arrangement for selectivelyheating a reflective STOF film to accomplish internal patterning;

FIGS. 8A-C are schematic top views of different second zones of aninternally patterned optical film, and superimposed thereon possiblepaths of a light beam relative to the film capable of forming thedepicted zones;

FIG. 9A is an idealized plot showing the relative intensity of a beam oflight as a function of the depth as the light beam propagates into thefilm, with three curves provided for three different optical films;

FIG. 9B is an idealized plot showing a local absorption coefficient as afunction of the depth or axial position within the film, with threecurves corresponding to the three curves of FIG. 9A; and

FIGS. 10-12 are a schematic side or sectional views of various articlesthat each include a STOF reflective film attached to a mask having astructured surface.

In the figures, like reference numerals designate like elements.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In FIG. 1A we see a schematic side or sectional view of componentsinvolved in various steps of a process that uses a reflective STOF filmas a mask for patterning another article. In a first step, a reflectiveSTOF film 110 is provided. The film 110 may initially be substantiallyspatially uniform, i.e., it may exhibit a first reflectivecharacteristic over its entire useable area. Details of the wide varietyof available STOF film types are provided elsewhere herein, e.g., in thediscussion below. Next, the film 110 is internally patterned bydirecting a suitable first radiant beam 112 onto selected portions ofthe film. By “internally patterned” here, we mean that a pattern of anydesired shape is imparted to the film by virtue of a change to one ormore interior layers of the film (e.g. where the film has two outerlayers and one or more interior layers sandwiched between the outerlayers) and/or to one or more materials of the film that residesubstantially inside the film, rather than a change in surface texture,roughness, or other surface effects of the film. The pattern may be ofany desired shape. For example, the pattern may be a grid or array ofsmall rectangles or other shapes that correspond to a pixel array forgiven type of liquid crystal display (LCD) device. An optional lens 114or other focusing element or system may be used to increase the flux ofthe radiant beam in the selected portions. If desired, beam deflectors,moveable stages, and/or other scanning equipment can be used to scan theradiant beam across the portions of the STOF film. By proper selectionor adjustment of the intensity, wavelength, and other significantproperties of the beam 112, in combination with proper tailoring of theabsorptive characteristics of the STOF film 110, the reflectivecharacteristics of the film 110 can be changed in the selected portionsof the film, referred to here as a treated second zone 110 b or area,relative to neighboring first zones or areas 110 a, 110 c. Thus, in thefirst zones 110 a, 110 c, the original first reflective characteristicof the film 110 is maintained, while in the second zone 110 b, adifferent second reflective characteristic is provided. The change fromthe first reflective characteristic to the second reflectivecharacteristic is, in exemplary cases, substantially attributable to achange in birefringence of at least a portion of the first film, and notsubstantially attributable to any substantial change or damage to astructural integrity or morphology of the film.

The patterned STOF film 110 may then be placed in proximity to anotherarticle 116 that can also be patterned with radiant energy.Alternatively, the film 110 may be placed at a location that is distantfrom article 116, and a focusing system such as one or more lenses maybe used to image the film 110 onto the article 116. Note that theradiant energy used to pattern the article 116 preferably differs insome way from the first radiant beam used to pattern the STOF film.Initially, the article 116 may have a first detectable characteristicthat may be substantially spatially uniform over its entire useablearea. The first detectable characteristic may be optical in nature,e.g., relating to transmission, reflection, absorption, scattering,polarization, wavelength/color, or the like, or it may be different,e.g., electrical, mechanical, and/or chemical in nature, or combinationsthereof.

A second radiant beam 118 may then be provided to illuminate the article116 through the patterned film 110, which now functions as a mask. Asmentioned above, the radiant beam 118 preferably differs from the firstradiant beam 112 that was used to pattern the STOF film. One or both ofthe first and second radiant beams may utilize polarized light, and thepolarization states may be different. In order to avoid parallax orspreading effects from the mask to the article 116, it may beparticularly helpful to configure the second beam 118 as a collimated orpartially collimated beam of light. Furthermore, the angular andintensity distribution of the second radiant beam, the geometricalarrangement of the patterned film 110 relative to the article 116, andother pertinent processing considerations may be taken into account whenpatterning the film 110 so as to ensure proper feature size andorientation in the final patterned article 116.

The second reflective characteristic may be more reflective than thefirst reflective characteristic, or vice versa. For purposes of FIG. 1A,we assume that the second reflective characteristic of film 110, in thesecond zone 110 b, is more reflective and less transmissive of thesecond radiant beam 118 than the first reflective characteristic in thefirst zones 110 a, 110 c. For this reason, the portion 118 a of beam 118that impinges upon the second zone is shown as being reflected. In analternative embodiment, the second reflective characteristic in zone 110b may be less reflective and more transmissive than the first reflectivecharacteristic in the first zones 110 a, 110 c. The first or secondreflective characteristic, which may be specular or diffuse depending onthe design of the STOF film, may have a relatively high reflectivity forthe second radiant beam, and/or at another wavelength of interest, e.g.,a reflectivity of at least 90%, or at least 95%, or at least 99%.

Radiation from the second radiant beam 118 that passes through thepatterned STOF film 110 and impinges upon the article 116 is tailored tochange the first detectable characteristic to a different seconddetectable characteristic. By virtue of the masking provided by thepatterned STOF film, this change in the article 116 is accomplished onlyin selected locations, which in this case are zones 116 a, 116 c ofarticle 116, the zones 116 a, 116 c corresponding substantially to zones110 a, 110 c of film 110. Zone 116 b of article 116 remains untreated bythe beam 118 due to the assumed high reflectivity of the beam at zone110 b of film 110.

In a final step, the patterned article 116 can be removed and put to usein a suitable end-use application for which it is designed.

The article 116 may be or comprise a block or other solid form, a plate,a film, and/or the like, and may be or comprise another STOF film. Thearticle 116 may be or comprise, for example, a layer of photoresistdisposed on a semiconductor substrate or another substrate, or a layerof photo-alignable liquid crystal material, or a layer of curablematerial. The detectable characteristic of the article may be altered bythe incidence and absorption of light, e.g. suitable ultraviolet,visible, or infrared light. For example, the change in the detectablecharacteristic may involve a curing operation (e.g. UV curing) or otherchemical reaction triggered by the second radiant beam. The change indetectable characteristic may also involve phase transformations and/orchemical decomposition. Molecular orientation processes such as stressrelaxation may also be involved. Such processes can result in changes inbirefringence of the article 116. The masking effect provided bypatterned STOF film 110 provides spatially selective reflectivity of thesecond radiant beam 118, thus providing control over the locations atwhich such changes in the article 116 are permitted, and in some cases,control over the extent of such changes (e.g. the degree of change of agiven characteristic) at any of those locations.

The article 116 may also be or comprise a composite construction ofvarious layers or other components. If the components of such an article116 are arranged in a layered configuration, the article may have atleast one outer or upper layer capable of being processed by the secondradiant beam, and may have a blocking layer (such as any of the blockinglayers disclosed in PCT Publication WO 2010/075373 (Merrill et al.),“Multilayer Optical Films Suitable for Bi-Level Internal Patterning”)disposed underneath the processable layer so as to protect still deeperlayers or components from the effects of the second radiant beam and/orother radiant beams.

If the article 116 is a flexible film, then tension and/or roll supportand/or planar support can be used to reduce vibrations, chatter, and/orundulation during processing. If the article 116 is a roll of film, theprocess can be continuous or semi-continuous. In cases where requiredtolerances are not too stringent, a roll of unpatterned STOF film usedfor the article 116 can be unwound into a processing line that isequipped with a patterned STOF film that functions as a mask. The STOFmask can be contacted under controlled tension to the unpatterned STOFfilm, with or without an intervening release liner, and the films canthen be conveyed together into a radiant processing unit. One or both ofthese films can be supported by a moving belt through the processingunit. If both a top and bottom belt are used, then at least one shouldbe reasonably transmissive to the processing light (i.e., the secondradiant beam). Alternatively, the unpatterned STOF film or other article116 can be conveyed separately into the radiant processing unit, andthen secured and registered with the STOF mask in a continuous orsemi-continuous fashion. For example, the conveyance may stop forsecuring, registering, and/or radiantly processing the article 116, andthen restart so as to carry the processed article 116 out of theprocessing unit and carry the next (unprocessed) article 116 into theprocessing unit. For example, a portion of a film roll can be laminatedto a glass plate. The film can then be cut and conveyed, or conveyedwhile still attached to the plate, into the processing unit. In thatunit, the STOF mask may be brought into contact with the laminatedfilm/plate, or may be maintained at a controlled distance from suchlaminated article.

It may be advantageous in manufacturing to precisely write the initialpattern in the STOF mask 110 using one writing wavelength for the firstradiant beam, e.g. using a slow but precise laser scanning process, andthen using this precisely written STOF mask to selectively block thesecond radiant beam at a different writing wavelength in order topattern the article 116, e.g. using a simpler, faster process. Forexample, treating a STOF film using a scanning laser may create aprecise pattern, but may also be slower and more costly thanpattern-wise treating a film using a non-scanning or non-patterninglight source, such as a flashlamp, in combination with a mask. The maskcan be written with a scanning laser, for example, on a STOF film. ThisSTOF mask can then be used as a spatial filter for a wide beam of lightfrom a flashlamp or other suitable radiant source to process otherpatternable articles 116. Consider a process that uses a STOF mask incombination with a second radiant beam having a wavelength in a nearinfrared band, e.g. at or near 1064 nm, to pattern-wise process apatternable article 116. The article 116 may be or comprise a STOF filmthat reflects light over a portion of the visible band, e.g., red,green, or blue light, e.g. for application in a display, and thatabsorbs light at 1064 nm. Processing of this film by heat-inducingabsorption at 1064 nm may convert the visible red, green, or bluereflectivity to a window-like appearance, for example, in treated areas.A STOF mask 110 used to pattern such an article 116 may possess areflection band that overlaps or encompasses the 1064 nm wavelength ofthe second radiant beam, and may also have a substantial absorption overa different wavelength band, e.g. in the visible, ultraviolet, or otherportion of the infrared. A first radiant beam at this differentwavelength band is used in an earlier step to pattern the STOF mask 110,i.e., to eliminate its reflection band at 1064 nm in selected zones. Inanother example, the article 116 may be pattern-wise processed using asecond radiant beam having a near-ultraviolet wavelength and a STOF maskthat selectively reflects such near-ultraviolet wavelength. The STOFmask itself may have been processed or treated using a first radiantbeam having a visible or infrared wavelength, for example.

Thus, a STOF film may be pattern-wise treated by the rapid absorption ofradiant energy in a first wavelength band of interest, e.g. through theincorporation of an absorbing dye or other agent that absorbs at thefirst wavelength band, or using an intrinsic absorption of a materialused in the construction, so as to produce a patterned mask. Thispatterned mask, which may reflect light at a second wavelength band insome places and transmit such light in other places, may subsequently beused, in combination with a radiant source emitting at the secondwavelength band, to treat the article 116. The article 116 may includean absorbing dye or other agent that absorbs at the second wavelengthband significantly more than the patterned mask at this wavelength.Light from the radiant source that passes through the patterned mask andimpinges on the article 116 can be used for the radiant processing ofthe article.

Using a reflective STOF film as a mask for the patterning of otherarticles has several possible advantages, some of which may depend upondetails of implementation. First, the ability to use laser scanning towrite the pattern into the STOF film mask allows precise, fine detail inthe subsequently processed or patterned article. Second, the STOF filmmask may be mounted on a transparent support member, such as a glassplate, to maintain precise dimensional control of the mask. Third, theSTOF film mask may be patterned after being mounted to such a supportmember. Fourth, the patterning of the STOF film mask may also be carriedout to create alignment marks (e.g., optical “holes” or “blocks”) thatcan be used for registration with corresponding marks or features thatmay be present on the article to ensure proper registration between themask and the patternable article. In the case of a display, the marks orfeatures may be the certain pixels themselves, turned on. A passive oractive optical sensor, e.g. a sensor that utilizes a laser whose radiantoutput (characterized by wavelength, intensity, polarization, angle ofincidence, and so forth) is ineffective to treat or modify both the maskand the patternable article, may be used for alignment purposes. Ifdesired, the STOF film mask may be designed to include two differentoptical packets of microlayers: one to reflect the second radiant beam(used to pattern the article 116), and one to use for opticalregistration purposes. The STOF film mask may be or comprise a bi-levelpatterned reflective film as described, for example, in PCT PublicationWO 2010/075373 (Merrill et al.), “Multilayer Optical Films Suitable forBi-Level Internal Patterning”, to achieve the desired effect.

For maximum utility, it may be desirable for the STOF film mask tomaintain the fidelity of its precise pattern of reflective andtransmissive zones even after many uses, e.g., after many exposures to asecond radiant beam used to process patternable articles 116. Thelifetime of the mask will be a function of its environmental conditions,including thermal and incident radiation conditions. In some cases itmay be desirable to fabricate multiple generations of STOF masks. Forexample, a primary or master STOF mask may be formed using a controlledscanning process in order. This primary mask may then be used to make alimited set, e.g., one or more, of secondary STOF masks. Each suchsecondary STOF mask may be used to make as many patternable articles aspossible before the secondary STOF mask becomes degraded, after whichthe degraded secondary STOF mask may be replaced with a different(unused) secondary STOF mask, and so forth. The primary or master STOFmask thus allows for the fabrication of limited lifetime secondary STOFmasks, which in turn may be used to process the patternable articles.

In some cases, the mask may become part of a finished article. Forexample, in a finished article intended for use in the visiblewavelength region, such as a security film, reflective portions of theSTOF film mask may reflect at wavelengths outside of the visible band.For example, such a film mask may have a normal incidence reflectionband in the ultraviolet region, or in a region above about 900-1000 nm,depending on requirements for off-axis (oblique angle) viewing whichcauses the reflection bands to shift to shorter wavelengths. If the STOFfilm mask is intended to be substantially transparent over its entireuseable area over all visible wavelengths and if the mask includes a(spatially patterned) reflection band in the infrared region, then thepacket(s) of microlayers providing that infrared reflection band woulddesirably be designed with an optical repeat unit tailored to suppresshigher order reflection peaks. The finished article may include such aSTOF mask film together with a patternable article, such as a secondSTOF film, disposed below or behind the STOF film mask. The second STOFfilm may exhibit a different reflection band, e.g. patterned reflectionat visible wavelengths. The STOF film mask of such a finished articlemay be patterned first by exposing the finished article to a firstradiant beam, e.g. a first scanned laser beam. Whether or not thespatial pattern of this STOF film mask is visible to the unaided eye,the STOF mask so patterned will allow for the later pattern-wisetreatment of the second STOF film using a second radiant beam, where thesecond radiant beam may be non-scanning and may impinge upon the entireuseable area of the STOF mask but (due to the spatial filtering actionof the STOF mask) only selected portions of the second STOF film. An enduser viewing the resulting finished article may observe the patterningof the second STOF film but may be unable to detect or observe thepatterning of the STOF mask, which may remain in the construction.

In some cases, a patterned STOF film that is part of a finished articlemay be treated with a radiant beam a second time (and if desired, third,fourth, and more times) to modify the initial pattern that was producedin the STOF film to pattern the patternable article (e.g. article 116 inFIG. 1A). Modification of the initial pattern in the STOF film mayinclude forming a second pattern in the STOF film by heat treatingselected areas or zones of the STOF film that were not heat-treated inthe initial patterning step while allowing other areas or zones of theSTOF film that were not heat-treated in the initial patterning step toremain untreated. Modification of the initial pattern may also includeheat-treating the entire useable area of the STOF film, or at leastenough of the useable area of the STOF film, so that substantially nopattern remains in the STOF film, i.e., so that substantially the entireuseable area of the STOF film has an optical characteristic (e.g. areflective characteristic) associated with the reduced birefringenceassociated with the heat treatment.

Thus, for example, a STOF film may be patterned with a first patternusing a first radiant beam, e.g., using a scanning process or using amasking process such as that of FIG. 1A or 1B. Areas or zones of theSTOF film that were treated with the first radiant beam exhibit a secondreflective characteristic different from an original first reflectivecharacteristic of the STOF film. This patterned STOF film may be part ofa composite article that includes another patternable article (e.g.article 116 in FIG. 1A), or the patterned STOF film may later beattached to the patternable article to form the composite article. Thecomposite article may then be exposed to a second radiant beam, suchthat the patterned STOF film masks portions of the second radiant beamso as to pattern the patternable article. This may, for example,substantially transfer the first pattern from the STOF film to thepatternable article. Thereafter, a third radiant beam, which may besimilar to the first radiant beam except that it is allowed to impingeupon at least some areas of the STOF film that the first radiant beamdid not impinge upon, so that at least these areas of the STOF film alsoexhibit a change from the first reflective characteristic to the secondreflective characteristic. If the third radiant beam does not treat allpreviously untreated areas of the STOF film, then a second pattern,different from the first pattern, is provided in the STOF film. On theother hand, if the third radiant beam does treat all previouslyuntreated areas of the STOF film (e.g. in cases where the third radiantbeam impinges upon substantially the entire STOF film), then the STOFfilm becomes substantially unpatterned with a reflective characteristiccharacterized by the second reflective characteristic. Note that,depending upon the design of the STOF film, the second reflectivecharacteristic may either be more reflective, or less reflective, thanthe original first reflective characteristic. The re-treatment of theSTOF film by the third radiant beam may be used as ananti-counterfeiting or anti-tampering measure.

In some cases, the second radiant beam in combination with the STOF maskmay change the detectable characteristic of the patternable article(e.g. article 116 in FIG. 1A) in a way that is temporary and/orreversible. Consider again the case of a composite article that includesboth a STOF film mask and a patternable article. A detectablecharacteristic of the patternable article is temporarily changed byexposing the patternable article to a second radiant beam through theSTOF film mask. Portions of the second radiant beam that impinge uponportions of the patternable article may optically reveal the pattern ofthe mask in the patternable article, e.g., at a different wavelengththan that of the second radiant beam. For example, consider a STOF maskthat reflects UV light in some zones and transmits UV light in otherzones. Such a pattern of reflective and transmissive zones may have beenprocessed in the STOF mask using, e.g., a first radiant beam at aninfrared wavelength. The patternable article beneath or behind the STOFfilm mask may be a film that contains a fluorescent dye or similarsubstance that is excited at UV wavelengths and that emits at visiblewavelengths. Exposing the composite article to a second radiant beam ofUV light allows the UV light to impinge upon only those portions of thepatternable article that lie directly beneath transparent zone(s) of theSTOF mask. A pattern of visible fluorescent light can thus be providedby the composite article. Note that this pattern is temporary andreversible, because after the second radiant beam is turned off orotherwise removed, the fluorescent light pattern will disappear.

Embodiments that utilize a STOF film mask may also utilize a blockinglayer configured to block the wavelength of light used in the firstradiant beam to pattern the STOF film mask, and/or to block otherwavelengths of light. For example, a second radiant beam may include notonly light at a second wavelength that is effective to pattern thepatternable article beneath the STOF film, but may also include light ata third wavelength capable of changing the reflective characteristics ofthe STOF film mask or otherwise degrading the STOF film mask. In such acase, a blocking layer that reflects light at the third wavelength buttransmits light at the second wavelength may be provided as a protectivelayer above or in front of the STOF film mask, between the mask and thesource of the second radiant beam. The blocking layer may also desirablytransmit light at a first wavelength, at which the STOF film mask isprocessed or patterned. Such a blocking layer may be or comprise, forexample, a mirror-like multilayer optical film (MOF) whose reflectionband covers the third wavelength or at least a portion of amask-sensitive wavelength band. Such a film may be added atop the STOFfilm mask.

A reflective STOF film may be applied to a film or article containing animage or other pattern that can be observed or detected using a machineto provide useful information. Before or after the STOF film is appliedto such patterned article, a first reflective characteristic of the STOFfilm may be selectively patterned into a different second reflectivecharacteristic using a first radiant beam. The first radiant beam may“turn on” a reflectivity of the STOF film, i.e., the second reflectivecharacteristic may be more reflective than the first reflectivecharacteristic (e.g., a window to mirror, or window to polarizer, orpolarizer to mirror characteristic), or the first radiant beam may “turnoff” a reflectivity of the STOF film, i.e., the second reflectivecharacteristic may be less reflective than the first reflectivecharacteristic (e.g., a mirror to window, or polarizer to window, ormirror to polarizer characteristic). A third radiant beam may later beused to eliminate or modify the pattern of the STOF film by treatingsome or all of the areas on the patterned STOF film still having thefirst reflective characteristic so that they change to the secondreflective characteristic as a result of heat-induced reduction ofbirefringence in the STOF film. The re-treatment of the STOF by thethird radiant beam may make the useful information in the underlyingfilm or article more accessible (e.g. if the second reflectivecharacteristic is less reflective than the first reflectivecharacteristic) or less accessible (e.g. if the second reflectivecharacteristic is more reflective than the first reflectivecharacteristic). The re-patterned STOF film may thus fully or partiallymask the information in the underlying patterned film or article.

FIG. 1B is a schematic side or sectional view of components involved invarious steps of a process in which a mask 120 is used to pattern areflective STOF film 122. The mask 120 may be or comprise a mask ofconventional design, e.g., a simple transparent film on which an opaquematerial such as ink is printed in a pattern, or it may be or comprisean internally patterned STOF film. For example, the mask 120 may be thepatterned STOF film 110 of FIG. 1A, or the patterned article 116 of FIG.1A. In any case, the mask 120 is patterned such that it includes firstareas or zones 120 a, 120 c that substantially block (e.g., absorband/or reflect) light, and other zones 120 b that substantially transmitlight. The pattern may be of any desired shape. For example, the patternmay be a grid or array of small rectangles or other shapes thatcorrespond to a pixel array for given type of liquid crystal display(LCD) device.

In a first step, the mask 120 is placed in proximity to the reflectiveSTOF film 122, or the mask 120 may be imaged onto the film 122 asexplained above. The STOF film 122 may initially be substantiallyspatially uniform, i.e., it may exhibit a first reflectivecharacteristic over its entire useable area. Details of the wide varietyof available STOF film types are provided elsewhere herein, e.g., in thediscussion below.

Next, the STOF film 122 is internally patterned by providing a firstradiant beam 124 to illuminate the film 122 through the mask 120. Thespatial filtering provided by the mask 120 causes the first radiant beam124 to impinge upon only selected portions of the film. By properselection or adjustment of the intensity, wavelength, and othersignificant properties of the beam 124, in combination with propertailoring of the absorptive characteristics of the STOF film 122, thereflective characteristics of the film 122 can be changed in theselected portions of the film, referred to here as a treated second zoneor area 122 b, relative to neighboring first zones or areas 122 a, 122c. Thus, in the first zones 122 a, 122 c, the original first reflectivecharacteristic of the film 122 is maintained, while in the second zone122 b, a different second reflective characteristic is provided. Thechange from the first reflective characteristic to the second reflectivecharacteristic is, in exemplary cases, substantially attributable to achange in birefringence of at least a portion of the first film, and notsubstantially attributable to any substantial change or damage to astructural integrity (e.g., layer structure or immiscible blendmorphology) of the film.

In order to avoid parallax or spreading effects from the mask to thefilm 122, it may be particularly helpful to configure the radiant beam124 as a collimated or partially collimated beam of light. Furthermore,the angular and intensity distribution of the radiant beam, thegeometrical arrangement of the mask 120 relative to the STOF film 122,and other pertinent processing considerations may be taken into accountwhen designing the pattern for mask 120 so as to ensure proper featuresize and orientation in the STOF film 122. Since the radiant beam 124may be a stationary, large area, and relatively uniform beam rather thana smaller beam that is scanned across the useable area of the film,substantially all areas of the STOF film 122 that undergo a change inreflective characteristic may experience such change at substantiallythe same time. Similar observations also apply to the radiant beam 118of FIG. 1A.

In a final step, the patterned STOF film 122 can be removed and put touse in a suitable end-use application for which it is designed.

Before considering other combinations of masks and STOF films, some ofwhich are discussed later in connection with FIGS. 10-12, we now turn toFIGS. 2 through 9B to provide further information and backgroundregarding STOF films and the capability of processing them in such a wayas to change their reflective characteristics or other opticalcharacteristics using selective heating provided by the localizedabsorption of radiant energy, the selective heating giving rise to arelaxation in birefringence of at least one constituent material orlayer of the film. FIG. 2 is a perspective view of a roll of reflectiveSTOF film 210 that has been internally patterned or spatially tailoredusing spatially selective birefringence reduction of at least some ofthe internal layers or materials (not shown in FIG. 2) to providedifferent reflective characteristics in different portions or zones ofthe film so as to form indicia. The internal patterning defines distinctzones 212, 214, 216 that are shaped so as to form the indicia “3M” asshown. The film 210 is shown as a long flexible material wound into aroll because the methodology described herein is advantageouslycompatible with high volume roll-to-roll processes. However, themethodology is not limited to flexible roll goods and can be practicedon small piece parts or samples as well as non-flexible films andarticles.

The reflectivity of the film 210 may be specular in nature, e.g., asprovided by a multilayer optical film having packets of generally planarmicrolayers, or it may be diffuse in nature, e.g. as provided by ablended layer having at least a first and second material arranged indistinct first and second phases in the blended layer. The reflectivitymay also depend on polarization state of the light.

The “3M” indicia is visible or otherwise detectable because thedifferent zones 212, 214, 216 have different reflective characteristics.In the depicted embodiment, zone 212 has a first reflectivecharacteristic and zones 214, 216 have a second reflectivecharacteristic different from the first reflective characteristic. Insome cases, the film 210 may be at least partially light transmissive.In such cases, and where the film 210 has different reflectivities inits zones 212, 214, 216, those zones will also have differenttransmissive characteristics that correspond to their respectivereflective characteristics. In general, of course, transmission (T) plusreflection (R) plus absorption (A)=100%, or T+R+A=100%. When dealingwith films that may appreciably diffusely scatter the transmitted and/orreflected light, we keep in mind that T may represent the hemispherictransmission, i.e., all light that exits the film on a side of the filmopposite the light source, regardless of its propagation directionwithin a solid angle of 27 c, and R may likewise represent thehemispheric reflection, i.e., all light that exits the film on the sameside of the film as the light source, regardless of its propagationdirection within a complementary 27 c solid angle. In some embodimentsthe film is composed entirely of materials that have low absorption overat least a portion of the wavelength spectrum. This may be the case evenfor films that incorporate an absorbing dye or pigment to promote heatdelivery, since some absorbing materials are wavelength specific intheir absorptivity. For example, infrared dyes are available thatselectively absorb in the near-infrared wavelength region but that havevery little absorption in the visible spectrum. At the other end of thespectrum, many polymer materials that are considered to be low loss inthe optical film literature do have low loss over the visible spectrumbut also have significant absorption at certain ultraviolet wavelengths.Thus, in many cases the film 210 may have an absorption that is small ornegligible over at least a limited portion of the wavelength spectrum,such as the visible spectrum, in which case the reflection andtransmission over that limited range take on a complementaryrelationship because T+R=100%−A, and since A is small,T+R≈100%.

As mentioned elsewhere herein, the different reflective characteristicsof the film 210 in the different patterned zones are each the result ofstructural features (such as a stack of microlayers in a multilayeroptical film, or distinct first and second phases in a blended layer)that are internal to the film, rather than the result of coatingsapplied to the surface of the film or other surface features. Thisaspect of the disclosed films makes them advantageous for securityapplications (e.g. where the film is intended for application to aproduct, package, or document as an indicator of authenticity) becausethe interior features are difficult to copy or counterfeit.

The first and second reflective characteristics differ in some way thatis perceptible under at least some viewing conditions to permitdetection of the pattern by an observer or by a machine. In some casesit may be desirable to maximize the difference between the first andsecond reflective characteristics at visible wavelengths so that thepattern is conspicuous to human observers under most viewing andlighting conditions. In other cases it may be desirable to provide onlya subtle difference between the first and second reflectivecharacteristics, or to provide a difference that is conspicuous onlyunder certain viewing conditions. In either case the difference betweenthe first and second reflective characteristics is preferablyattributable primarily to difference in the refractive index propertiesof interior features of the optical film in the different neighboringzones of the film, and is not primarily attributable to differences inthickness between the neighboring zones.

The zone-to-zone differences in refractive index can produce variousdifferences between the first and second reflective characteristicsdepending on the design of the optical film. In some cases the firstreflective characteristic may include a first reflection band with agiven center wavelength, band edge, and maximum reflectivity, and thesecond reflective characteristic may differ from the first by having asecond reflection band that is similar in center wavelength and/or bandedge to the first reflection band, but that has a substantiallydifferent maximum reflectivity (whether greater or lesser) than thefirst reflection band, or the second reflection band may besubstantially absent from the second reflection characteristic. Thesefirst and second reflection bands may be associated with light of onlyone polarization state, or with light of any polarization statedepending on the design of the film.

In embodiments that include a diffusely reflective blended layer, thefirst reflective characteristic may be or include, for example, aminimum, maximum, or average diffuse reflectivity (or transmission)value over the visible wavelength range, where the reflectivity (ortransmission) may be measured for an incident beam of a specifiedpolarization state and for reflected (or transmitted) light within aspecified solid angle of reflected (or transmitted) directions relativeto the incident beam, or within a hemispheric (27 c) solid angle on theincident light-side (or the opposite side) of the film, for example. Thesecond reflective characteristic may differ from the first by having asubstantially different (whether greater or lesser) minimum, maximum, oraverage reflectivity or transmission value for the same specifiedincident light and measurement conditions as the first characteristic.Furthermore, one of the first and second characteristics may correspondsubstantially to a highly transmissive, low scattering appearance as inthe case of a window film, at least for incident light of onepolarization state.

Thus, for example, the first reflective characteristic (which may bediffuse or specular in nature), in zone 212, may have a peak or averagereflectivity of R₁ in a wavelength range of interest for a specifiedcondition of incident light (e.g. a specified direction, polarization,and wavelength, such as normally incident unpolarized visible light, ornormally incident visible light polarized along a particular in-planedirection). The reduced birefringence in the zones 214, 216 yields asecond reflective characteristic (which may again be diffuse or specularin nature), such as a different peak or average reflectivity of R₂ inthe same wavelength range of interest for the same specified conditionof incident light. R₁ and R₂ are compared under the same illuminationand observation conditions, for example, R₁ and R₂ may be measured ashemispheric reflectivity on the incident light-side of the film, for thespecified incident condition. If R₁ and R₂ are expressed in percentages,R₂ may differ from R₁ by at least 10%, or by at least 20%, or by atleast 30%. As a clarifying example, R₁ may be 70%, and R₂ may be 60%,50%, 40%, or less. Alternatively, R₁ may be 10%, and R₂ may be 20%, 30%,40%, or more. R₁ and R₂ may also be compared by taking their ratio. Forexample, R₂/R₁ or its reciprocal may be at least 2, or at least 3.

In some cases the first and second reflective characteristics may differin their dependence of reflectivity with viewing angle. For example, thefirst reflective characteristic may include a first reflection band thathas a given center wavelength, band edge, and maximum reflectivity atnormal incidence, and the second reflective characteristic may include asecond reflection band that is very similar to these aspects of thefirst reflection band at normal incidence. With increasing incidenceangle, however, although both the first and second reflection bands mayshift to shorter wavelengths, their respective maximum reflectivitiesmay deviate from each other greatly. For example, the maximumreflectivity of the first reflection band may remain constant orincrease with increasing incidence angle, while the maximum reflectivityof the second reflection band, or at least the p-polarized componentthereof, may decrease with increasing incidence angle, e.g. in a rangefrom normal incidence to the Brewster's angle.

In embodiments that include at least one multilayer optical film, thedifferences discussed above between the first and second reflectivecharacteristics may relate to reflection bands that cover a portion ofthe visible spectrum. Such differences may in those cases be perceivedas differences in color between the different in-plane zones of thefilm.

A first reflective characteristic may have a given minimum, maximum, oraverage reflectivity or transmission for light of a given polarizationstate normally incident on the film, and a second reflectivecharacteristic may have the same or similar reflectivity or transmissionvalue for light of the same incidence conditions. With increasingincidence angle, however, the value may increase for the firstcharacteristic and decrease for the second characteristic, or viceversa, or the value may remain relatively constant for onecharacteristic and substantially increase or decrease for the other. Inembodiments that include at least one diffusely reflective blendedlayer, different first and second diffusely reflective characteristicsmay exhibit the same or similar average reflectivity over visiblewavelengths for normally incident light of a given polarization state,but as the incidence angle increases, the average reflectivity of thefilm in a first zone (corresponding to the first diffusely reflectivecharacteristic) may increase, while the average reflectivity of the filmin a second zone (corresponding to the second diffusely reflectivecharacteristic) may decrease.

Turning now to FIG. 3A, we see there a portion of a multilayer film 310,which may be a STOF film, in schematic side view to reveal the structureof the film including its interior layers. Such a film may be used as ablocking layer in the disclosed embodiments, and, if it is made to havesuitable absorptive characteristics, may also be used as a patternablereflector or STOF film in the disclosed embodiments. The film is shownin relation to a local x-y-z Cartesian coordinate system, where the filmextends parallel to the x- and y-axes, and the z-axis is perpendicularto the film and its constituent layers and parallel to a thickness axisof the film. Note that the film 310 need not be entirely flat, but maybe curved or otherwise shaped to deviate from a plane, and even in thosecases arbitrarily small portions or regions of the film can beassociated with a local Cartesian coordinate system as shown. The film310 may be considered to represent the film 210 of FIG. 2 in any of itszones 212, 214, 216, since the individual layers of the film 210preferably extend continuously from each such zone to the next.

Multilayer optical films include individual layers having differentrefractive indices so that some light is reflected at interfaces betweenadjacent layers. These layers, sometimes referred to as “microlayers”,are sufficiently thin so that light reflected at a plurality of theinterfaces undergoes constructive or destructive interference to givethe multilayer optical film the desired reflective or transmissiveproperties. For multilayer optical films designed to reflect light atultraviolet, visible, or near-infrared wavelengths, each microlayergenerally has an optical thickness (a physical thickness multiplied byrefractive index) of less than about 1 μm. However, thicker layers canalso be included, such as skin layers at the outer surfaces of themultilayer optical film, or protective boundary layers (PBLs) disposedwithin the multilayer optical film to separate coherent groupings (knownas “stacks” or “packets”) of microlayers. In FIG. 3A, the microlayersare labeled “A” or “B”, the “A” layers being composed of one materialand the “B” layers being composed of a different material, these layersbeing stacked in an alternating arrangement to form optical repeat unitsor unit cells ORU 1, ORU 2, . . . ORU 6 as shown. Typically, amultilayer optical film composed entirely of polymeric materials wouldinclude many more than 6 optical repeat units if high reflectivities aredesired. Note that all of the “A” and “B” microlayers shown in FIG. 3Aare interior layers of film 310, except for the uppermost “A” layerwhose upper surface in this illustrative example coincides with theouter surface 310 a of the film 310. The substantially thicker layer 312at the bottom of the figure can represent an outer skin layer, or a PBLthat separates the stack of microlayers shown in the figure from anotherstack or packet of microlayers (not shown). If desired, two or moreseparate multilayer optical films can be laminated together, e.g. withone or more thick adhesive layers, or using pressure, heat, or othermethods to form a laminate or composite film.

In some cases, the microlayers can have thicknesses and refractive indexvalues corresponding to a ¼-wave stack, i.e., arranged in optical repeatunits each having two adjacent microlayers of equal optical thickness(f-ratio=50%, the f-ratio being the ratio of the optical thickness of aconstituent layer “A” to the optical thickness of the complete opticalrepeat unit), such optical repeat unit being effective to reflect byconstructive interference light whose wavelength λ is twice the overalloptical thickness of the optical repeat unit, where the “opticalthickness” of a body refers to its physical thickness multiplied by itsrefractive index. In other cases, the optical thickness of themicrolayers in an optical repeat unit may be different from each other,whereby the f-ratio is greater than or less than 50%. In the embodimentof FIG. 3A, the “A” layers are depicted for generality as being thinnerthan the “B” layers. Each depicted optical repeat unit (ORU 1, ORU 2,etc.) has an optical thickness (OT₁, OT₂, etc.) equal to the sum of theoptical thicknesses of its constituent “A” and “B” layer, and eachoptical repeat unit reflects light whose wavelength λ is twice itsoverall optical thickness. The reflectivity provided by microlayerstacks or packets used in multilayer optical films in general, and inthe internally patterned multilayer optical films discussed herein inparticular, is typically substantially specular in nature, rather thandiffuse, as a result of the generally smooth well-defined interfacesbetween microlayers, and the low haze materials that are used in atypical construction. In some cases, however, the finished article maybe tailored to incorporate any desired degree of scattering, e.g., usinga diffuse material in skin layer(s) and/or PBL layer(s), and/or usingone or more surface diffusive structures or textured surfaces, forexample.

In some embodiments, the optical thicknesses of the optical repeat unitsin a layer stack may all be equal to each other, to provide a narrowreflection band of high reflectivity centered at a wavelength equal totwice the optical thickness of each optical repeat unit. In otherembodiments, the optical thicknesses of the optical repeat units maydiffer according to a thickness gradient along the z-axis or thicknessdirection of the film, whereby the optical thickness of the opticalrepeat units increases, decreases, or follows some other functionalrelationship as one progresses from one side of the stack (e.g. the top)to the other side of the stack (e.g. the bottom). Such thicknessgradients can be used to provide a widened reflection band to providesubstantially spectrally flat transmission and reflection of light overthe extended wavelength band of interest, and also over all angles ofinterest. Thickness gradients tailored to sharpen the band edges at thewavelength transition between high reflection and high transmission canalso be used, as discussed in U.S. Pat. No. 6,157,490 (Wheatley et al.)“Optical Film With Sharpened Bandedge”. For polymeric multilayer opticalfilms, reflection bands can be designed to have sharpened band edges aswell as “flat top” reflection bands, in which the reflection propertiesare essentially constant across the wavelength range of application.Other layer arrangements, such as multilayer optical films having2-microlayer optical repeat units whose f-ratio is different from 50%,or films whose optical repeat units include more than two microlayers,are also contemplated. These alternative optical repeat unit designs canbe configured to reduce or to excite certain higher-order reflections,which may be useful if the desired reflection band resides in or extendsto near infrared wavelengths. See, e.g., U.S. Pat. No. 5,103,337(Schrenk et al.) “Infrared Reflective Optical Interference Film”, U.S.Pat. No. 5,360,659 (Arends et al.) “Two Component Infrared ReflectingFilm”, U.S. Pat. No. 6,207,260 (Wheatley et al.) “Multicomponent OpticalBody”, and U.S. Pat. No. 7,019,905 (Weber) “Multi-layer Reflector WithSuppression of High Order Reflections”.

Adjacent microlayers of the multilayer optical film have differentrefractive indices so that some light is reflected at interfaces betweenadjacent layers. We refer to the refractive indices of one of themicrolayers (e.g. the “A” layers in FIG. 3A) for light polarized alongprincipal x-, y-, and z-axes as n1 x, n1 y, and n1 z, respectively. Thex-, y-, and z-axes may, for example, correspond to the principaldirections of the dielectric tensor of the material. Typically, and fordiscussion purposes, the principle directions of the different materialsare coincident, but this need not be the case in general. We refer tothe refractive indices of the adjacent microlayer (e.g. the “B” layersin FIG. 3A) along the same axes as n2 x, n2 y, n2 z, respectively. Werefer to the differences in refractive index between these layers as Δnx(=n1 x−n2 x) along the x-direction, Δny (=n1 y−n2 y) along they-direction, and Δnz (=n1 z−n2 z) along the z-direction. The nature ofthese refractive index differences, in combination with the number ofmicrolayers in the film (or in a given stack of the film) and theirthickness distribution, controls the reflective and transmissivecharacteristics of the film (or of the given stack of the film) in agiven zone. For example, if adjacent microlayers have a large refractiveindex mismatch along one in-plane direction (Δnx large) and a smallrefractive index mismatch along the orthogonal in-plane direction(Δnyz≈0), the film or packet may behave as a reflective polarizer fornormally incident light. In this regard, a reflective polarizer may beconsidered for purposes of this application to be an optical body thatstrongly reflects normally incident light that is polarized along onein-plane axis (referred to as the “block axis”) if the wavelength iswithin the reflection band of the packet, and strongly transmits suchlight that is polarized along an orthogonal in-plane axis (referred toas the “pass axis”). “Strongly reflects” and “strongly transmits” mayhave different meanings depending on the intended application or fieldof use, but in many cases a reflective polarizer will have at least 70,80, or 90% reflectivity for the block axis, and at least 70, 80, or 90%transmission for the pass axis.

For purposes of the present application, a material is considered to be“birefringent” if the material has an anisotropic dielectric tensor overa wavelength range of interest, e.g., a selected wavelength or band inthe UV, visible, and/or infrared portions of the spectrum. Stateddifferently, a material is considered to be “birefringent” if theprincipal refractive indices of the material (e.g., n1 x, n1 y, n1 z)are not all the same. The “birefringence” of a given material or layermay then refer to the difference between its maximum principalrefractive index and its minimum principal refractive index, unlessotherwise indicated. Negligible amounts of birefringence can generallybe ignored. In the case of a blended layer for a diffusely reflectivefilm, a constituent material in the continuous phase preferably exhibitsa birefringence of at least 0.03, or 0.05, or 0.10. In some cases, thebirefringence of any given material or layer may be specified to be atleast 0.02, or 0.03, or 0.05, for example.

In another example, adjacent microlayers may have a large refractiveindex mismatch along both in-plane axes (Δnx large and Δny large), inwhich case the film or packet may behave as an on-axis mirror. In thisregard, a mirror or mirror-like film may be considered for purposes ofthis application to be an optical body that strongly reflects normallyincident light of any polarization if the wavelength is within thereflection band of the packet. Again, “strongly reflecting” may havedifferent meanings depending on the intended application or field ofuse, but in many cases a mirror will have at least 70, 80, or 90%reflectivity for normally incident light of any polarization at thewavelength of interest. In variations of the foregoing embodiments, theadjacent microlayers may exhibit a refractive index match or mismatchalong the z-axis (Δnz≈0 or Δnz large), and the mismatch may be of thesame or opposite polarity or sign as the in-plane refractive indexmismatch(es). Such tailoring of Δnz plays a key role in whether thereflectivity of the p-polarized component of obliquely incident lightincreases, decreases, or remains the same with increasing incidenceangle. In yet another example, adjacent microlayers may have asubstantial refractive index match along both in-plane axes(Δnxz≈Δnyz≈0) but a refractive index mismatch along the z-axis (Δnzlarge), in which case the film or packet may behave as a so-called“p-polarizer”, strongly transmitting normally incident light of anypolarization, but increasingly reflecting p-polarized light ofincreasing incidence angle if the wavelength is within the reflectionband of the packet.

In view of the large number of permutations of possible refractive indexdifferences along the different axes, the total number of layers andtheir thickness distribution(s), and the number and type of microlayerpackets included in the multilayer optical film, the variety of possiblemultilayer optical films 310 and packets thereof is vast. Exemplarymultilayer optical films are disclosed in: U.S. Pat. No. 5,486,949(Schrenk et al.) “Birefringent Interference Polarizer”; U.S. Pat. No.5,882,774 (Jonza et al.) “Optical Film”; U.S. Pat. No. 6,045,894 (Jonzaet al.) “Clear to Colored Security Film”; U.S. Pat. No. 6,179,949(Merrill et al.) “Optical Film and Process for Manufacture Thereof”;U.S. Pat. No. 6,531,230 (Weber et al.) “Color Shifting Film”; U.S. Pat.No. 6,939,499 (Merrill et al.) “Processes and Apparatus for MakingTransversely Drawn Films with Substantially Uniaxial Character”; U.S.Pat. No. 7,256,936 (Hebrink et al.) “Optical Polarizing Films withDesigned Color Shifts”; U.S. Pat. No. 7,316,558 (Merrill et al.)“Devices for Stretching Polymer Films”; PCT Publication WO 2008/144136A1 (Nevitt et al.) “Lamp-Hiding Assembly for a Direct Lit Backlight”;PCT Publication WO 2008/144656 A2 (Weber et al.) “Backlight and DisplaySystem Using Same”.

We note that at least some of the microlayers in at least one packet ofthe multilayer optical film are birefringent in at least one zone of thefilm (e.g., zones 212, 214, 216 of FIG. 2). Thus, a first layer in theoptical repeat units may be birefringent (i.e., n1 x≠n1 y, or n1 x≠n1 z,or n1 y≠n1 z), or a second layer in the optical repeat units may bebirefringent (i.e., n2 x≠n2 y, or n2 x≠n2 z, or n2 y≠n2 z), or both thefirst and second layers may be birefringent. Moreover, the birefringenceof one or more such layers is diminished in at least one zone relativeto a neighboring zone. In some cases, the birefringence of these layersmay be diminished to zero, such that they are optically isotropic (i.e.,n1 x=n1 y=n1 z, or n2 x=n2 y=n2 z) in one of the zones but birefringentin a neighboring zone. In cases where both layers are initiallybirefringent, depending on materials selection and processingconditions, they can be processed in such a way that the birefringenceof only one of the layers is substantially diminished, or thebirefringence of both layers may be diminished.

Exemplary multilayer optical films are composed of polymer materials andmay be fabricated using a variety of flow processes, includingcoextrusion, film casting, and film stretching or drawing processes.Typically, birefringence is developed in at least some layers throughone or more of these various flow processes. Reference is made to U.S.Pat. No. 5,882,774 (Jonza et al.) “Optical Film”, U.S. Pat. No.6,179,949 (Merrill et al.) “Optical Film and Process for ManufactureThereof”, and U.S. Pat. No. 6,783,349 (Neavin et al.) “Apparatus forMaking Multilayer Optical Films”. The multilayer optical film may beformed by co-extrusion of the polymers as described in any of theaforementioned references. The polymers of the various layers may bechosen to have similar rheological properties, e.g., melt viscosities,so that they can be co-extruded without significant flow disturbances.Extrusion conditions are chosen to adequately feed, melt, mix, and pumpthe respective polymers as feed streams or melt streams in a continuousand stable manner. Temperatures used to form and maintain each of themelt streams may be chosen to be within a range that avoids freezing,crystallization, or unduly high pressure drops at the low end of thetemperature range, and that avoids material degradation at the high endof the range.

In brief summary, the fabrication method of a multilayer optical filmmay comprise: (a) providing at least a first and a second stream ofresin corresponding to the first and second polymers to be used in thefinished film; (b) dividing the first and the second streams into aplurality of layers using a suitable feedblock, such as one thatcomprises: (i) a gradient plate comprising first and second flowchannels, where the first channel has a cross-sectional area thatchanges from a first position to a second position along the flowchannel, (ii) a feeder tube plate having a first plurality of conduitsin fluid communication with the first flow channel and a secondplurality of conduits in fluid communication with the second flowchannel, each conduit feeding its own respective slot die, each conduithaving a first end and a second end, the first end of the conduits beingin fluid communication with the flow channels, and the second end of theconduits being in fluid communication with the slot die, and (iii)optionally, an axial rod heater located proximal to said conduits; (c)passing the composite stream through an extrusion die to form amultilayer web in which each layer is generally parallel to the majorsurface of adjacent layers; and (d) casting the multilayer web onto achill roll, sometimes referred to as a casting wheel or casting drum, toform a cast multilayer film. This cast film may have the same number oflayers as the finished film, but the layers of the cast film aretypically much thicker than those of the finished film. Furthermore, thelayers of the cast film are typically all isotropic.

Many alternative methods of fabricating the cast multilayer web can alsobe used. One such alternative method that also utilizes polymercoextrusion is described in U.S. Pat. No. 5,389,324 (Lewis et al.).

After cooling, the multilayer web can be drawn or stretched to producethe near-finished multilayer optical film, details of which can be foundin the references cited above. The drawing or stretching accomplishestwo goals: it thins the layers to their desired final thicknesses, andit orients the layers such that at least some of the layers becomebirefringent. The orientation or stretching can be accomplished alongthe cross-web direction (e.g. via a tenter), along the down-webdirection (e.g. via a length orienter), or any combination thereof,whether simultaneously or sequentially. If stretched along only onedirection, the stretch can be “unconstrained” (wherein the film isallowed to dimensionally relax in the in-plane direction perpendicularto the stretch direction) or “constrained” (wherein the film isconstrained and thus not allowed to dimensionally relax in the in-planedirection perpendicular to the stretch direction). If stretched alongboth in-plane directions, the stretch can be symmetric, i.e., equalalong the orthogonal in-plane directions, or asymmetric. Alternatively,the film may be stretched in a batch process. In any case, subsequent orconcurrent draw reduction, stress or strain equilibration, heat setting,and other processing operations can also be applied to the film.

Turning now to FIG. 3B, we see there a portion of a blended layer of adiffusely reflective optical film 320, which may be a STOF film, inschematic perspective view to reveal the interior structure or blendmorphology of the layer/film. We refer to the film as a diffuselyreflective optical film even in cases where the film may have a hightransparency with little or no haze, i.e., where it has a window-likeappearance, so long as such film was derived from, or can be processedinto, a film that diffusely reflects or diffusely transmits light of agiven incidence direction and polarization state in accordance with theselective heating techniques set forth herein. The film 320 is shown inrelation to a local x-y-z Cartesian coordinate system, where the filmextends parallel to the x- and y-axes, and the z-axis is perpendicularto the film and parallel to a thickness axis of the film. Note that thefilm 320 need not be entirely flat, but may be curved or otherwiseshaped to deviate from a plane, and even in those cases arbitrarilysmall portions or regions of the film can be associated with a localCartesian coordinate system as shown. The film 320 may in general beconsidered to represent the film 210 of FIG. 2 in any of its zones 212,214, 216, since the film 210 may be or include a blended layer thatextends continuously from each such zone to the next. As depicted, film320 includes a first light-transmissive polymer or other material whichis in the form of a continuous or matrix phase 322, and a secondlight-transmissive polymer or other material which is in the form of adiscontinuous or disperse phase 324.

Many different materials may be used to fabricate the disclosed opticalfilms, depending on the specific application to which the optical filmis directed. Such materials may include inorganic materials such assilicon-based polymers, organic materials such as liquid crystals, andpolymeric materials, including monomers, copolymers, grafted polymers,and mixtures or blends thereof. The exact choice of materials for agiven application will be driven by the desired match and/or mismatchobtainable in the refractive indices of the different phases along aparticular axis, as well as the desired physical properties in theresulting product. In cases where one of the materials is present in theblended layer in a continuous phase, such material will generally becharacterized by being substantially transparent in the region of thespectrum desired, and such material desirably exhibits birefringence atleast prior to the selective heat treatment discussed herein.

At least some of the diffusely reflective films disclosed herein, and/orthe blended layers thereof, may be composed substantially entirely ofpolymeric materials, although in some cases non-polymeric materials mayalso be used. In some cases, only two different polymeric materials maybe used, but in other cases more than two such polymeric materials maybe used.

In general, the class of optical films formed with co-extrudable blendsof thermoplastics is of particular interest. With these systems, filmsmay be formed, oriented by one or more stretching processes, and woundinto roll stock for later use. The stretching process thereby impartsthe birefringence in at least one continuous phase. Thermoplasticsprovide distinct advantages over systems comprising thermosets that mustbe cured prior to winding into a roll. For example, thermoplastics mayallow post-processing shaping, e.g. through thermoforming methods. Therolls may also be treated later for spatial patterning. Some suitablematerials for use are discussed, for example in U.S. Pat. No. 5,882,774(Ouderkirk et al.), U.S. Pat. No. 6,179,948 (Merrill et al.), U.S. Pat.No. 6,673,275 (Allen et al.), U.S. Pat. No. 7,057,816 (Allen et al.), aswell as U.S. Patent Application Publications US 2004/0164434 (Tabar etal.) and US 2008/0020186 (Hebrink et al.). With regard to the continuousphase, the various polyestersand their co-polymers described in thesereferences, including in particular polyethylene terephthalate (PET),polyethylene naphthalate (PEN), and copolymers of PEN and PET, areparticularly useful, especially the so-called “coPENs.” With regard tothe at least one other phase, whether dispersed or co-continuous, thepolystyrenes, polyacrylates, and polycarbonates described in thesereferences are particularly useful.

A further consideration in the choice of materials is that the resultingproduct desirably contains at least two distinct phases in order to formthe microscopic structures within the blended layer that can provide thedesired scattering. This may be accomplished by casting the opticalmaterial from two or more materials which are immiscible with eachother. Alternatively, if it is desired to make an optical material witha first and second material which are not immiscible with each other,and if the first material has a higher melting point than the secondmaterial, in some cases it may be possible to embed particles ofappropriate dimensions of the first material within a molten matrix ofthe second material at a temperature below the melting point of thefirst material. The resulting mixture can then be cast into a film, withsubsequent and/or simultaneous orientation, to produce an orientedoptical film or body. In another variation, immiscible materials thatreact,e.g. by transesterfication, can be used to form the distinctphases, if the extrusion processing times are short enough and thetemperatures low enough to maintain immiscible blocks. In still anothervariation, a third component, e.g. another polymer such as a blockco-polymer, or a so-called “compatiblizer”, can be added to help controlthe interfacial tension or other characteristics and thus also the sizeand shape distributions of the blended phases.

The materials selected for use in the disclosed films, and the degree oforientation of these materials, may in some cases be chosen so that thedifferent materials in the blended layer of the finished film, whetherin a heat-treated zone thereof or in a zone that has not been heattreated, have at least one axis for which the associated indices ofrefraction are substantially equal. The match of refractive indicesassociated with that axis, which typically, but not necessarily, is anaxis transverse to the direction of orientation, results insubstantially no reflection of light in that plane of polarization.

At least a first material (e.g. in the form of a disperse phase) mayexhibit a decrease in the refractive index associated with the directionof orientation after stretching. If a second material (e.g. in the formof a continuous phase) is positive, a negative strain inducedbirefringence of the first material has the advantage of increasing thedifference between indices of refraction of the adjoining phasesassociated with the orientation axis while the reflection of light withits plane of polarization perpendicular to the orientation direction maystill be negligible. If a reflective polarizer is desired, differencesbetween the indices of refraction of adjoining phases in the in-planedirection orthogonal to the orientation direction are desirably lessthan about 0.05, or 0.03, or 0.02, or 0.01 after orientation.

The material in the form of a disperse phase may also exhibit a positivestrain-induced birefringence. However, this can be altered by means ofheat treatment to match the refractive index of the axis perpendicularto the orientation direction of the other material (e.g. in the form ofa continuous phase). The temperature of the heat treatment should not beso high as to relax the birefringence in the continuous phase.

The size of the structures or features in the disperse phase also canhave a significant effect on scattering. If the disperse phase particlesare too small (e.g., less than about 1/30 the wavelength of light in themedium of interest) and if there are many particles per cubicwavelength, the optical body may behave as a medium with an effectiveindex of refraction somewhat between the indices of the two phases alongany given axis. In such a case, very little light is scattered. If theparticles are very large, the number of particles that can beaccommodated per unit volume of the blended layer becomes low, and lightmay be specularly reflected from the surface of the particle, with verylittle diffusion or scattering into other directions. If such very largeparticles become disk-shaped or flattened along the x- and y-directions,iridescence effects (which may or may not be desirable) may occur.Practical limits may also be reached when particles become large in thatthe thickness of the optical body becomes greater and desirablemechanical properties are compromised.

The dimensions of the particles of the disperse phase after alignmentcan be tailored depending on the desired use of the optical material.Thus, for example, the dimensions of the particles may be tailoreddepending on the wavelength of electromagnetic radiation that is ofinterest in a particular application, with different dimensions requiredfor reflecting or transmitting visible, ultraviolet, infrared, andmicrowave radiation. Generally, however, the length of the particlesshould be such that they are approximately greater than the wavelengthof electromagnetic radiation of interest in the medium, divided by 30.

In applications where the optical body is to be used as a low lossreflective polarizer, the particles may have a length that is greaterthan about 2 times the wavelength of the electromagnetic radiation overthe wavelength range of interest, and preferably over 4 times thewavelength. The average diameter of the particles may be equal to orless than the wavelength of the electromagnetic radiation over thewavelength range of interest, and preferably less than 0.5 of thedesired wavelength. While the dimensions of the disperse phase are asecondary consideration in most applications, they become of greaterimportance in thin film applications, where there is comparativelylittle diffuse reflection.

While in many cases the refractive index mismatch may be the predominantfactor relied upon to promote scattering (e.g., a diffuse mirror orpolarizer film may have a substantial mismatch in the indices ofrefraction of the continuous and disperse phases along at least onein-plane axis), changes to the geometry of the particles of the dispersephase may also have an effect (e.g. a secondary effect) on scattering.Thus, the depolarization factors of the particles for the electric fieldin the index of refraction match and mismatch directions can reduce orenhance the amount of scattering in a given direction. For example, whenthe disperse phase is elliptical in a cross-section taken along a planeperpendicular to the axis of orientation (see e.g. disperse phase 324 inFIG. 3B), the elliptical cross-sectional shape of the disperse phase cancontribute to asymmetric diffusion in both back-scattered light andforward-scattered light. The effect can either add to or detract fromthe amount of scattering caused by the refractive index mismatch, buttypically has a relatively small influence on scattering.

The shape of the disperse phase particles can also influence the degreeof diffusion of light scattered from the particles. This shape effect istypically small but increases as the aspect ratio of the geometricalcross-section of the particle in the plane perpendicular to thedirection of incidence of the light increases and as the particles getrelatively larger. It is often desirable for the disperse phaseparticles to be sized less than several wavelengths of light in one ortwo mutually orthogonal dimensions if diffuse, rather than specular,reflection is desired.

For a low loss reflective polarizer, the film may consist of a dispersephase disposed within the continuous phase as a series of rod-likestructures which, as a consequence of orientation, have a high aspectratio which can enhance reflection for polarizations parallel to theorientation direction by increasing the scattering strength anddispersion for that polarization relative to polarizations perpendicularto the orientation direction. However, the particles or structures ofthe disperse phase may be provided with many different geometries. Thus,the disperse phase may be disk-shaped or elongated disk-shaped, orrod-shaped, or spherical. The disperse phase particle may be a disk as aresult of the film being significantly oriented or stretched in both thex- and y-directions, but the disk may be elongated along the y-directiondue to a greater degree of orientation in that direction. Alternatively,the disk may be substantially symmetrical due to approximately equaldegrees of orientation in the x- and y-directions. Alternatively, thedisk may be elongated along the x-direction due to a greater degree oforientation in that direction. Other embodiments are contemplatedwherein the disperse phase has cross sections which are approximatelyelliptical (including circular), polygonal, irregular, or a combinationof one or more of these shapes. The cross-sectional shape and size ofthe particles of the disperse phase may also vary from one particle toanother, or from one region of the film to another (i.e., as a functionof depth from the surface to the core).

Besides a continuous/disperse phase combination, the different polymersthat make up the blended layer of the diffusely reflective film mayalternatively be arranged in a co-continuous phase relationship. Furtherdetails of co-continuous phase constructions can be found e.g. in U.S.Pat. No. 7,057,816 (Allen et al.). A co-continuous phase constructionmay be provided in which the two phases are fibrillar and form aninterpenetrating polymer network (IPN). The fibers may be randomlyoriented, or oriented along a given axis. Other co-continuous systemsmay comprise an open-celled matrix of a first material (first phase),with a second material disposed in a co-continuous manner (second phase)within the cells of the matrix.

The different materials used in the different phases of the diffuselyreflective optical films have different refractive indices along aparticular direction or axis, whether in a heat-treated zone thereof orin a zone that has not been heat treated, so that some light polarizedalong such direction or axis is reflected at interfaces between theadjacent phases, and collectively scattered. We may refer to therefractive indices of a first material in the blended layer (e.g., inFIG. 3B, the first light-transmissive polymer in the form of continuousphase 322) for light polarized along principal x-, y-, and z-axes as n1x, n1 y, and n1 z, respectively. The x-, y-, and z-axes may, forexample, correspond to the principal directions of the dielectric tensorof the material. Typically, and for discussion purposes, the principaldirections of the different materials in the blended layer arecoincident, but this need not be the case in general. We refer to therefractive indices of a second material (adjacent the first material) inthe blended layer (e.g., in FIG. 3B, the second light-transmissivepolymer or other material which is in the form of a discontinuous ordisperse phase 324) along the same axes as n2 x, n2 y, n2 z,respectively. We refer then to differences in refractive index betweenthese materials or phases as Δnx (=n1 x−n2 x) along the x-direction, Δny(=n1 y−n2 y) along the y-direction, and Δnz (=n1 z−n2 z) along thez-direction. The nature of these refractive index differences, incombination with the thickness, composition (e.g. volume fraction of thefirst and second materials in the blended layer), and blend morphology(e.g., the size, shape, and distribution of structures of the firstpolymer and structures of the second polymer in the blended layer) ofthe blended layer, controls the reflective and transmissivecharacteristics of the such layer, in a given zone. For example, ifadjacent phases have a large refractive index mismatch along onein-plane direction (Δnx large) and a small refractive index mismatchalong the orthogonal in-plane direction (Δnyz≈0), the film or blendedlayer may behave as a diffusely reflective polarizer for normallyincident light. In this regard, a diffusely reflective polarizer may beconsidered for purposes of this application to be an optical body thatstrongly diffusely reflects normally incident light that is polarizedalong one in-plane axis (referred to as the “block axis”), and stronglytransmits such light that is polarized along an orthogonal in-plane axis(referred to as the “pass axis”). “Strongly reflects” and “stronglytransmits” may have different meanings depending on the intendedapplication or field of use, but in many cases a diffusely reflectivepolarizer will have at least 70, 85, 90, or 95% reflectivity for theblock axis, and at least 70, 80, or 85% transmission for the pass axis.These reflectivity and transmission values are assumed to include theeffects of Fresnel reflection at the outer surfaces (air/polymerinterfaces) of the film.

In another example, adjacent phases may have a large refractive indexmismatch along both in-plane axes (Δnx large and Δny large), in whichcase the film or blended layer may behave as an on-axis diffuse mirror.In this regard, a diffuse mirror or mirror-like film may be consideredfor purposes of this application to be an optical body that stronglydiffusely reflects normally incident light of any polarization. Again,“strongly diffusely reflecting” may have different meanings depending onthe intended application or field of use, but in many cases a diffusemirror will have at least 70, 80, or 90% reflectivity for normallyincident light of any polarization at the wavelength of interest.

In variations of the foregoing embodiments, the adjacent phases mayexhibit a refractive index match or mismatch along the z-axis (Δnz≈0 orΔnz large), and the mismatch may be of the same or opposite polarity orsign as the in-plane refractive index mismatch(es). Such tailoring ofΔnz plays a key role in whether the reflectivity of the p-polarizedcomponent of obliquely incident light increases, decreases, or remainsthe same with increasing incidence angle. In yet another example,adjacent phases may have a substantial refractive index match along bothin-plane axes (Δnxz≈Δnyz≈0) but a refractive index mismatch along thez-axis (Δnz large), in which case the film or layer may behave as aso-called “p-polarizer”, strongly transmitting normally incident lightof any polarization, but increasingly reflecting p-polarized light ofincreasing incidence angle.

There are a large number of permutations of possible refractive indexdifferences between adjacent phases along the different axes, possiblethicknesses of the blended layer, possible compositions of the blendedlayer, and possible blend morphologies of the blended layer. Hence, thevariety of possible diffusely reflective films and blended layersthereof is vast. Exemplary diffusely reflective optical films thatcomprise at least one blended layer are disclosed in U.S. Pat. No.5,825,543 (Ouderkirk et al.), U.S. Pat. No. 6,179,948 (Merrill et al.),and U.S. Pat. No. 7,057,816 (Allen et al.).

At least one of the materials that form one of the phases in the blendedlayer of the optical film is birefringent in at least one zone of thefilm (e.g., zones 212, 214, 216 of FIG. 2). Thus, a first phase in theblended layer may be birefringent (i.e., n1 x≠n1 y, or n1 x≠n1 z, or n1y≠n1 z), or a second phase in the blended layer may be birefringent(i.e., n2 x≠n2 y, or n2 x≠n2 z, or n2 y≠n2 z), or both the first andsecond phases may be birefringent. Moreover, the birefringence of one ormore such phases is diminished in at least one zone relative to aneighboring zone. In some cases, the birefringence of these phase(s) maybe diminished to zero, such that it or they are optically isotropic(i.e., n1 x=n1 y=n1 z, or n2 x=n2 y=n2 z) in one of the zones butbirefringent in a neighboring zone. In cases where both phases areinitially birefringent, depending on materials selection and processingconditions, they can be processed in such a way that the birefringenceof only one of the phases is substantially diminished, or thebirefringence of both phases may be diminished.

Exemplary diffusely reflective optical films are composed ofthermoplastic polymer materials and may be fabricated by co-extrusion,film casting, and orienting processes. Reference is made to U.S. Pat.No. 6,179,949 (Merrill et al.) “Optical Film and Process for ManufactureThereof”. The optical film may be formed by co-extrusion of the polymersas described in any of the aforementioned references. For example, thepolymers may be dried prior to processing to reduce degradation, fedsimultaneously, in measured proportions, into an extruder (of eithersingle screw or twin screw configuration with or without applied vacuum)through a melt train with appropriate filters as desired, spread in adie manifold and exited through a die orifice onto a quench wheel orinto a quenching nip roll system. The polymers of the various layers maybe chosen to have rheological properties, e.g., melt viscosities, sothat the scale of the phases is adequately through the action of theflow. For example, increasing the ratio of a continuous phase viscosityto a dispersed phase viscosity can increase the elongation and break-upof the dispersed phase into smaller droplets. An additionalcompatibilizer or stabilizing component may be added to reduce theinterfacial tensions between the respect phases, thereby reducing thesurface tension driven tendency of the droplets to snap back into morespherical shapes or to re-aggregate or flocculate back into largerparticles. Extrusion conditions, including temperature, screw speeds,gear pump rates, etc., are chosen to adequately feed, melt, mix, andpump the polymers in a continuous and stable manner. Temperatures usedto form and maintain the melt stream may be chosen to be within a rangethat avoids freezing, crystallization, or unduly high pressure drops atthe low end of the temperature range, and that avoids materialdegradation at the high end of the range. High shear rates may be foundparticularly advantageous in processing in order to create fine-scalephase structures. In many cases, an increasing gradient in the scale ofthe phase structure may be found through the thickness of the blendlayer due to the decreasing shear field from the melt stream, e.g. die,walls to the flow stream center. Extensional flows can influence thephase sizes and shapes (blend morphology).

In many instances, a co-extrusion of multiple layers is desirable. Forexample, optically transparent, interior facilitation layers (e.g. acore layer, or set of layers) or outer skin layers, may be used, e.g. asdescribed in U.S. Pat. No. 6,179,948 (Merrill et al.). Blend layers mayalso comprise layers of a multilayer construction, e.g. formed usingprocessing methods described in U.S. Pat. No. 6,830,713 (Hebrink etal.). In some instances, the various alternating layers may comprisesimilar blend materials. In other instances, facilitation and blendlayers may alternate.

The film may then be formed, e.g. casting from a drop die onto aquenching wheel, e.g with electro-static pinning or between quenched niprolls, etc. to form the film, or the film can be formed onto a belt witha slot die and quenched. As described in U.S. Patent ApplicationPublication US 2008/0020186 (Hebrink et al.), the film may be partiallyoriented, e.g. by calendering, in the process of film forming. In somecases, a rolling bank configuration may be used with a calenderingprocess to further affect the phase sizes and shapes. In general, therate of quenching and the nature of the heat transfer from the outerfilm surfaces can impact the resulting blend morphology of the formedfilm.

After cooling, the web can be drawn or stretched to produce the nearfinished optical film, details of which can be found in the referencescited above. The drawing or stretching accomplishes two goals: itfurther orients and elongates the phases of the blend, and it orientsand imparts birefringence to at least one phase in at least one blendedlayer. Typically, at least one continuous phase acquires birefringencein this manner, although birefringence can also be imparted, in somecases, during the film forming step as previously described. Theorientation or stretching can be accomplished along the crosswebdirection (e.g. via a tenter), along the down-web direction (e.g. via alength orienter), or any combination thereof, whether simultaneously orsequentially. If stretched along only one direction, the stretch can be“unconstrained” (wherein the film is allowed to dimensionally relax inthe in-plane direction perpendicular to the stretch direction) or“constrained” (wherein the film is constrained and thus not allowed todimensionally relax in the in-plane direction perpendicular to thestretch direction). If stretched along both in-plane directions, thestretch can be symmetric, i.e., equal along the orthogonal in-planedirections, or asymmetric. The various stretching steps may also affectthe phases differently, e.g. as further described in U.S. Pat. No.6,179,948 (Merrill et al.). Alternatively, the film may be stretched ina batch process. In any case, subsequent or concurrent draw reduction,stress or strain equilibration, heat setting, and other processingoperations can also be applied to the film.

The multilayer optical films, diffusely reflective films, and otherdisclosed films and film bodies can also include additional layers andcoatings selected for their optical, mechanical, and/or chemicalproperties. For example, a UV absorbing layer can be added at one orboth major outer surfaces of the film to protect the film from long-termdegradation caused by UV light. Additional layers and coatings can alsoinclude scratch resistant layers, tear resistant layers, and stiffeningagents. See, e.g., U.S. Pat. No. 6,368,699 (Gilbert et al.).

In some cases, the natural or inherent absorptivity of one, some, or allof the constituent polymer materials that make up the STOF film may beutilized for the absorptive heating procedure. For example, manypolymers that are low loss over the visible region have substantiallyhigher absorptivity at certain ultraviolet wavelengths.

Exposing portions of the film to light of such wavelengths may be usedto selectively heat such portions of the film.

In other cases, absorbing dyes, pigments, or other agents can beincorporated into some or all of the individual layers or materials ofthe STOF film to promote absorptive heating as mentioned above. In somecases, such absorbing agents are spectrally selective, whereby theyabsorb in one wavelength region but not in another. For example, some ofthe disclosed films may be intended for use in the visible region, suchas with anti-counterfeiting security labels or as a component of aliquid crystal display (LCD) device or other display device, in whichcase an absorbing agent that absorbs at infrared or ultravioletwavelengths but not substantially at visible wavelengths may be used.Further, an absorbing agent may be incorporated into one or moreselected layers or materials of a film. For example, the film maycomprise two distinct microlayer packets separated by an optically thicklayer such as a protective boundary layer (PBL), a laminating adhesivelayer, one or more skin layers, or the like, and an absorbing agent maybe incorporated into one of the packets and not the other, or may beincorporated into both packets but at a higher concentration in onerelative to the other.

A variety of absorbing agents can be used. For optical films operatingin the visible spectrum, dyes, pigments, or other additives that absorbin the ultraviolet and infrared (including near infrared) regions may beused. In some cases it may be advantageous to select an agent thatabsorbs in a spectral range for which the polymer materials of the filmhave a substantially lower absorption. By incorporating such anabsorbing agent into selected layers of a multilayer optical film,directed radiation can preferentially deliver heat to the selectedlayers rather than throughout the entire thickness of the film.Exemplary absorbing agents may be melt extrudable so that they can beembedded into a selected layer set of interest. To this end, theabsorbers are preferably reasonably stable at the processingtemperatures and residence times required for extrusion. For furtherinformation on suitable absorbing agents, reference is made to U.S. Pat.No. 6,207,260 (Wheatley et al.) “Multicomponent Optical Body”.

FIG. 2 is a perspective view of a roll of reflective STOF film 210 thathas been internally patterned or spatially tailored using spatiallyselective birefringence reduction of at least some of the internallayers or materials (not shown in FIG. 2) to provide differentreflective characteristics in different portions or zones of the film soas to form indicia. The internal patterning defines distinct zones 212,214, 216 that are shaped so as to form the indicia “3M” as shown. Thefilm 210 is shown as a long flexible material wound into a roll becausethe methodology described herein is advantageously compatible with highvolume roll-to-roll processes. However, the methodology is not limitedto flexible roll goods and can be practiced on small piece parts orsamples as well as non-flexible films and articles.

We turn now to FIG. 4, which shows a schematic sectional view of aportion of the STOF film 210 of FIG. 2 in the vicinity of area 218 at aboundary of zone 212 and zone 216. In this expanded view of the film210, a narrow transition zone 215 can be seen separating the zone 212from the neighboring zone 216. Such a transition zone may or may not bepresent depending on processing details, and if it is not present thenzone 216 may be immediately adjacent to zone 212 with no significantintervening features. Construction details of film 210 can also be seen:the film includes optically thick skin layers 410, 412 on opposite sidesthereof, with a plurality of microlayers 414 and another plurality ofmicrolayers 416 disposed between the skin layers 410, 412. All of themicrolayers 414, 416 are interior to the film 210 by virtue of the outerskin layers. The space between microlayers 414 and 416 is left open inthe drawing, to allow for the case where the microlayers 414, 416 areportions of a single microlayer packet that begins at one skin layer 410and ends at the opposite skin layer 412, and also the case where themicrolayers 414, 416 are portions of two or more distinct microlayerpackets that are separated from each other by one or more opticallythick protective boundary layers (PBLs) or other optically thickinterior layer(s). In either case, the microlayers 414, 416 preferablyeach comprise two alternating polymer materials arranged into opticalrepeat units, each of the microlayers 414, 416 extending continuously ina lateral or transverse fashion from the zone 212 to the neighboringzone 216 as shown. The microlayers 414, 416 provide a first reflectivecharacteristic in the zone 212 by constructive or destructiveinterference, and at least some of the microlayers 414, 416 arebirefringent. The zones 215, 216 may have previously had the samecharacteristics as zone 212, but have been processed by the selectiveapplication of heat thereto in an amount sufficient to reduce oreliminate the birefringence of some of the microlayers 414, 416 in thezone 216 while maintaining the birefringence of the microlayers in zone212, the heat also being low enough to maintain the structural integrityof the microlayers 414, 416 in the treated zone 216. The reducedbirefringence of the microlayers 414, 416 in the zone 216 is primarilyresponsible for a second reflective characteristic for the zone 216 thatis different from the first reflective characteristic for the zone 212.

The film 210 has characteristic thicknesses d1, d2 in zone 212, andcharacteristic thicknesses d1 ‘, d2’ in zone 216, as shown in thefigure. The thicknesses d1, d1′ are physical thicknesses measured from afront outer surface of the film to a rear outer surface of the film inthe respective zones. The thicknesses d2, d2′ are physical thicknessesmeasured from the microlayer (at one end of a microlayer packet) that isdisposed nearest the front surface of the film to the microlayer (at anend of the same or a different microlayer packet) that is disposednearest the rear surface of the film. Thus, if one wishes to compare thethickness of the film 210 in zone 212 with the thickness of the film inzone 216, one may choose to compare d1 to d1′, or d2 to d2′, dependingon which measurement is more convenient. In most cases the comparisonbetween d1 and d1′ may well yield substantially the same result(proportionally) as the comparison between d2 and d2′. (Of course, incases where the film contains no outer skin layers, and where microlayerpackets terminate at both outer surfaces of the film, d1 and d2 becomethe same.) However, where a significant discrepancy exists, such aswhere a skin layer experiences a significant thickness change from oneplace to another but no corresponding thickness change exists in theunderlying microlayers, or vice versa, then it may be desirable to usethe d2 and d2′ parameters as being more representative of the overallfilm thickness in the different zones, in view of the fact that the skinlayers typically have a minor effect on the reflective characteristicsof the film compared to the microlayer packet(s).

Of course, for STOF films containing two or more distinct microlayerpackets separated from each other by optically thick layers, thethickness of any given microlayer packet can also be measured andcharacterized as the distance along the z-axis from the first to thelast microlayer in the packet. This information may become significantin a more in-depth analysis that compares the physical characteristicsof the film 210 in the different zones 212, 216.

As mentioned, the zone 216 has been treated with the selectiveapplication of heat to cause at least some of the microlayers 414, 416to lose some or all of their birefringence relative to theirbirefringence in neighboring zone 212, such that zone 216 exhibits areflective characteristic, resulting from constructive or destructiveinterference of light from the microlayers, that differs from areflective characteristic of zone 212. The selective heating process mayinvolve no selective application of pressure to zone 216, and it mayresult in substantially no thickness change (whether using theparameters d1/d1′ or the parameters d2/d2′) to the film. For example,the film 210 may exhibit an average thickness in zone 216 that deviatesfrom an average thickness in zone 212 by no more than the normalvariability in thickness that one observes in the zone 212, or in theuntreated film. Thus, the film 210 may exhibit in zone 212, or over anarea of the film encompassing a portion of zone 212 and zone 216 beforethe heat treatment of zone 216, a variability in thickness (whether d1or d2) of Δd, and the zone 216 may have spatially averaged thicknessesd1′, d2′ which differ from spatially averaged thicknesses d1, d2(respectively) in zone 212 by no more than Δd. The parameter Δd mayrepresent, for example, one, two, or three standard deviations in thespatial distribution of the thickness d1 or d2.

In some cases, the heat treatment of zone 216 may give rise to certainchanges to the thickness of the film in zone 216. These thicknesschanges may result from, for example, local shrinkage and/or expansionof the different materials that constitute the STOF film, or may resultfrom some other thermally-induced phenomenon. However, such thicknesschanges, if they occur, play only a secondary role in their effect onthe reflective characteristic of the treated zone 216 compared to theprimary role played by the reduction or elimination of birefringence inthe treated zone. Note also that in many cases it may be desirable tohold the film by its edges under tension during the selective heattreatment that accomplishes the internal patterning, in order to avoidwrinkling of the film, or for other reasons. The amount of tensionapplied and details of the heat treatment may also result in some amountof thickness change in the treated zones.

In some cases it is possible to distinguish the effect of a thicknesschange from a change in birefringence by analyzing the reflectiveproperties of the film. For example, if the microlayers in an untreatedzone (e.g. zone 212) provide a reflection band characterized by a leftband edge (LBE), right band edge (RBE), center wavelength λ_(c), andpeak reflectivity R₁, a given thickness change for those microlayers(with no change in the refractive indices of the microlayers) willproduce a reflection band for the treated zone having a peakreflectivity R₂ about the same as R₂, but having an LBE, RBE, and centerwavelength that are proportionally shifted in wavelength relative tothose features of the reflection band of the untreated zone, and thisshift can be measured. On the other hand, a change in birefringence willtypically produce only a very minor shift in wavelength of the LBE, RBE,and center wavelengths, as a result of the (usually very small) changein optical thickness caused by the change in birefringence. (Recall thatoptical thickness equals physical thickness multiplied by refractiveindex.) The change in birefringence can, however, have a large or atleast a significant effect on the peak reflectivity of the reflectionband, depending on the design of the microlayer stack. Thus, in somecases, the change in birefringence may provide a peak reflectivity R₂for the reflection band in the modified zone that differs substantiallyfrom R₁, where of course R₁ and R₂ are compared under the sameillumination and observation conditions. If R₁ and R₂ are expressed inpercentages, R₂ may differ from R₁ by at least 10%, or by at least 20%,or by at least 30%. As a clarifying example, R₁ may be 70%, and R₂ maybe 60%, 50%, 40%, or less. Alternatively, R₁ may be 10%, and R₂ may be20%, 30%, 40%, or more. R₁ and R₂ may also be compared by taking theirratio. For example, R₂/R₁ or its reciprocal may be at least 2, or atleast 3.

A significant change in peak reflectivity, to the extent it isindicative of a change in the interfacial reflectivity (sometimesreferred to as optical power) resulting from a change in refractiveindex difference between adjacent layers due to a change inbirefringence, is also typically accompanied by at least some change inthe bandwidth of the reflection band, where the bandwidth refers to theseparation between the LBE and RBE.

As we have discussed, in some cases the thickness of the film 210 in thetreated zone 216, i.e., d1′ or d2′, may differ somewhat from thethickness of the film in the untreated zone 212, even if no selectivepressure was in fact applied to the zone 216 during heat treatment. Forthis reason, FIG. 4 depicts d1′ as being slightly different from d1, andd2′ as being slightly different from d2. A transition zone 215 is alsoshown for generality, to show that a “bump” or other detectable artifactmay exist on the outer surface of the film as a consequence of theselective heat treatment. In some cases, however, the treatment mayresult in no detectable artifact between the neighboring treated anduntreated zones. For example, in some cases an observer who slides hisor her finger across the boundary between the zones may detect no bump,ridge, or other physical artifact between the zones.

Under some circumstances it is possible for thickness differencesbetween treated and untreated zones to be non-proportional through thethickness of the film. For example, in some cases it is possible for anouter skin layer to have a relatively small thickness difference,expressed as a percentage change, between the treated and untreatedzones, while one or more internal microlayer packets may have a largerthickness difference, also expressed as a percentage change, between thesame zones.

Although FIG. 4 shows the film 210 as comprising one or two microlayerpackets, in an alternative embodiment, these packets may be replacedwith one or two blended layers that provide diffusely reflectivecharacteristics. Each blended layer may include at least two polymermaterials that form two distinct phases, such as a continuous phase anda dispersed phase, or two co-continuous phases. At least one of thepolymer materials in a given blended layer may be birefringent in theuntreated zone 212 and is less birefringent (including e.g. isotropic)in the treated zone 216.

FIG. 5 shows a schematic sectional view of a portion of another STOFfilm 510 that incorporates internal patterning. Film 510 comprises outeroptically thick skin layers 512, 514, and a packet of microlayers thatreside in a stratum or layer 516 sandwiched between the skin layers. Allof the microlayers are internal to the film 510. (In alternativeembodiments, one or both skin layers may be omitted, in which case oneor both PBLs or outermost microlayers in the packet may become externallayers.) The microlayers include at least some microlayers that arebirefringent in at least some zones or areas of the film and that extendin a lateral or transverse fashion at least between neighboring zones ofthe film. The microlayers provide a first reflective characteristicassociated with constructive or destructive interference of light atleast in a first untreated zone 522 of the film. The film 510 has beenselectively heated in the neighboring zones 520, 524, without applyingany pressure selectively to these zones, so as to provide a secondreflective characteristic also associated with constructive ordestructive interference of light, but that differs from the firstreflective characteristic. These differences in reflectivecharacteristics may be manifested to an observer as differences in colorbetween the treated and untreated zones in reflected or transmittedlight. The respective colors and the differences therebetween alsotypically change or shift with angle of incidence. The film 510 may havesubstantially the same film thickness in the zones 520, 522, 524, or thefilm thickness may vary somewhat between these zones, but any differencein film thickness between the zones is not primarily responsible for thedifferences between the first and second reflective characteristics. Thezones 520, 522, 524 form a pattern that is internal or interior to thefilm, as indicated by the crosshatching in the stratum or layer 516. Thecrosshatching indicates that at least some of the microlayers in thecrosshatched region have a reduced birefringence (including zerobirefringence) compared to their birefringence in the zone 522 or inother untreated zones.

In an alternative embodiment, the packet of microlayers in layer 516 maybe replaced with a blended layer that includes at least two polymermaterials that form two distinct phases, such as a continuous phase anda dispersed phase, or two co-continuous phases. At least one of thepolymer materials in the blended layer may be birefringent in theuntreated zone 522 and is less birefringent (including e.g. isotropic)in the treated zones 520, 524, so that a first diffusely reflectivecharacteristic is provided in the untreated zone, and a different seconddiffusely reflective characteristic is provided in the treated zones.

In still other embodiments, internal patterning can be accomplishedindependently in two or more layers or levels within the STOF film. Atleast one blocking layer may also be provided between any two adjacentpatternable layers. The blocking layer may block a sufficient amount oflight at a write wavelength such that a first radiant beam comprisingthe write wavelength can be directed at a first zone of the STOF film tochange a first reflective characteristic of one layer (e.g. a packet ofmicrolayers or a blended layer with suitable absorptive characteristics)to a different second reflective characteristic, while not changing athird reflective characteristic of a second layer (e.g. another packetof microlayers or another blended layer with suitable absorptivecharacteristics) in the first zone. The blocking layer may also block asufficient amount of light at the write wavelength such that a secondradiant beam comprising the write wavelength can be directed at a secondzone of the film to change the third reflective characteristic of thesecond layer to a fourth reflective characteristic, while not changingthe first reflective characteristic of the first layer to the secondreflective characteristic in the second zone. The blocking layer mayachieve this functionality primarily by absorbing light at the writewavelength, by reflecting light at the write wavelength, or by acombination of absorbing and reflecting. Depending upon the design ofthe blocking layer and threshold characteristics of the respective firstand second patternable layers, the first and second radiant beams may beincident on the same side or major surface of the STOF film, or they maybe incident on opposite sides. In some designs, the first and secondradiant beams may also have different angles of incidence with respectto the film. For example, the first beam may be delivered atsubstantially normal incidence, and the second beam may be delivered ata large oblique angle relative to the film. Further informationregarding bi-level STOF films can be found in PCT Publication WO2010/075373 (Merrill et al.), “Multilayer Optical Films Suitable forBi-Level Internal Patterning”, and U.S. Application Ser. No. 61/360,127,“Retarder Film Combinations With Spatially Selective BirefringenceReduction”, filed Jun. 30, 2010.

FIGS. 5A-D help to explain the process of patterning a multilayeroptical film that is a STOF film. They also help explain some of thedifferent possible combinations of first and second reflectivecharacteristics in the untreated and treated zones, respectively, forany given writeable packet of microlayers. For descriptive purposes, thereflective characteristic of a reflective film, whether in a treated oruntreated zone, may be categorized into one of the following threetypes: mirror-like reflective characteristics, window-like reflectivecharacteristics, and polarizer-like reflective characteristics. Amirror-like reflective characteristic exhibits high reflectivity (e.g.,in some cases greater than 50%, 60%, 70%, 80%, 90%, 95%, or 99%) for allpolarization states of normally incident light, a window-like reflectivecharacteristic exhibits low reflectivity (e.g., in some cases less than20%, 10%, 5%, 3%, or 1%) for all polarization states of normallyincident light, and a polarizer-like reflective characteristic exhibitshigh reflectivity (e.g., in some cases greater than 50%, 60%, 70%, 80%,90%, 95%, or 99%) for normally incident light of one polarization stateand low reflectivity (e.g., in some cases less than 30%, 20%, 10%, 5%,3%, or 1%) for normally incident light of a different polarizationstate. (The reflective polarizer-like characteristic may alternativelybe expressed in terms of the difference in reflectivity of onepolarization state relative to the other polarization state.) Thereflectivity values discussed herein that are associated with multilayeroptical films or stacks may or may not include the Fresnel reflectionsat the outer air/polymer interfaces. For example, in some cases of highreflectivity these values may include the surface contributions, but insome cases of low reflectivity, these may exclude the surfacereflections. Reflectivity that includes the outer air/polymer surfacecontributions can be measured in a conventional manner using the barefilm immersed in air, while reflectivity that does not include theair/polymer surface contributions can be measured by use of an indexmatching fluid with cover layers of known reflectivity, and subtractingthe known reflectivity from the measurement.

The boundaries or limits of these different characteristics—e.g., whatis considered to be “high” reflectivity and what is considered to be“low” reflectivity—and the distinctions therebetween may depend on theend-use application and/or on system requirements. For example, amultilayer optical film, or a microlayer packet thereof, that exhibitsmoderate levels of reflectivity for all polarization states may beconsidered to be a mirror for purposes of some applications and a windowfor purposes of other applications.

Similarly, a multilayer optical film, or a microlayer packet thereof,that provides moderately different levels of reflectivity for differentpolarization states of normally incident light may be considered to be apolarizer for some applications, a mirror for other applications, and awindow for still other applications, depending on the exact reflectivityvalues and on the sensitivity of a given end-use application todifferences in reflectivity for different polarization states. Unlessotherwise indicated, the mirror, window, and polarizer categories arespecified for normally incident light. The reader will understand thatoblique-angle characteristics may in some cases be the same as orsimilar to, and in other cases may be drastically different from, thecharacteristics of an optical film at normal incidence.

In each of the graphs of FIGS. 5A-D, relative refractive index “n” isplotted on the vertical axis. On the horizontal axis, a position or markis provided for each of the six refractive indices that characterize atwo-layer optical repeat unit of a patternable multilayer optical film:“1 x”, “1 y”, and “1 z” represent the refractive indices of the firstlayer along the x-, y-, and z-axes, which were referred to above as n1x, n1 y, and n1 z. Likewise, “2 x”, “2 y”, and “2 z” represent therefractive indices of the second layer along the x-, y-, and z-axes,which were referred to above as n2 x, n2 y, and n2 z. Diamond-shapedsymbols (0) in the figures represent refractive indices of the materialsin a first processing stage. This first stage may correspond to polymerlayers that have been extruded and quenched or cast onto a castingwheel, for example, but that have not yet been stretched or otherwiseoriented. Open (unfilled) circle-shaped symbols (∘) in the figuresrepresent refractive indices of materials in a second stage ofprocessing, later than the first stage. The second stage may correspondto polymer layers that have been stretched or otherwise oriented into amultilayer optical film that reflects light by constructive ordestructive interference from interfaces between microlayers within thefilm. Small filled circle-shaped symbols or dots (•) in the figuresrepresent refractive indices of the materials in a third stage ofprocessing, later than the first and second stages. The third stage maycorrespond to polymer layers that, after being extruded and oriented,have been selectively heat treated, as discussed elsewhere herein. Suchheat treatment is typically limited to one or more particular portionsor zones of a film, referred to as treated zones.

By comparing the vertical coordinates of the various symbols in a givenfigure, the reader can readily ascertain a great deal of informationabout the multilayer optical film, its method of manufacture, and theoptical properties of its treated and untreated portions. For example,the reader can ascertain: if one or both material layers are or werebirefringent before or after the selective heat treatment, and whetherthe birefringence is uniaxial or biaxial, and whether the birefringenceis large or small. The reader can also ascertain from FIGS. 5A-Drelative magnitudes of each of the refractive index differences Δnx,Δny, Δnz between the two layers, for each of the three processing stages(cast state, stretched state, and treated state).

As discussed above, a precursor article to a finished, internallypatterned multilayer optical film can be a cast web of polymer material.The cast web may have the same number of layers as the finished film,and the layers may be composed of the same polymer materials as thoseused in the finished film, but the cast web is thicker and its layersare usually all isotropic. In some cases, however, not depicted in thefigures, the casting process may itself impart a level of orientationand birefringence in one or more of the materials. The diamond-shapedsymbols in FIGS. 5A-D represent the refractive indices of the twopolymer layers in the cast web that, after a subsequent stretchingprocedure, become the microlayers in the optical repeat units of themultilayer optical film. After stretching, at least some of the layersbecome oriented and birefringent, and an oriented (but stillunpatterned) multilayer optical film is formed. This is exemplified inFIGS. 5A-D by open circles that may be vertically displaced from theirrespective original values represented by the diamond-shaped symbols.For example, in FIG. 5A, the stretching procedure raises the refractiveindex of the second layer along the x-axis, but lowers its refractiveindex along the y- and z-axis. Such a refractive index shift may beobtained by suitably uniaxially stretching a positively birefringentpolymer layer along the x-axis while allowing the film to dimensionallyrelax along the y- and z-axes. In FIG. 5B, the stretching procedureraises the refractive index of the first layer along the x- and y-axes,but lowers its refractive index along the z-axis. Such a refractiveindex shift may be obtained by suitably biaxially stretching apositively birefringent polymer layer along the x- and y-axes. In FIG.5C, the stretching procedure raises the refractive index of both thefirst and second layers along the x-axis, lowers their respectiverefractive index along the z-axis, and maintains about the samerefractive index along the y-axis. In some cases, this refractive indexshift may be obtained by biaxially stretching a positively birefringentpolymer layer asymmetrically along the x- and y-axes, using a higherdegree of stretch along the x-axis compared to the y-axis. In othercases, this may be approximately obtained by uniaxially stretching alongan x-axis while constraining the film in the y-axis (constraineduniaxial stretching). Note that in FIGS. 5A and 5B, one of the layers inthe oriented but untreated state (open circles) is birefringent becauseat least two of the open circles (for n2 x, n2 y, and n2 z in FIG. 5A,and for n1 x, n1 y, and n1 z in FIG. 5B) have different values ofrefractive index n. In these depicted embodiments, the other polymerlayer remains isotropic after stretching as indicated by the samerefractive index values (n1 x=n1 y=n1 z in FIG. 5A, and n2 x=n2 y=n2 zin FIG. 5B) for the cast state and for the oriented but untreated state.

After formation of the at least partially birefringent multilayeroptical film having the microlayers arranged into optical repeat unitsto provide the first reflective characteristic, the film is ready forthe selective heating discussed above. The heating is carried outselectively in a second zone which neighbors a first zone of themultilayer optical film, and is tailored to selectively melt anddisorient in part or in whole at least one birefringent material in themicrolayer packet in order to reduce or eliminate the birefringence inat least some of the microlayers while leaving their birefringenceunchanged in the first (untreated) zone. The selective heating is alsocarried out to maintain the structural integrity of the layers in thesecond zone. If the birefringent material in the treated second zone isdisoriented in whole, i.e., completely, then the birefringentmicrolayers return to the isotropic state (e.g. of the cast web), whileremaining optically thin. This can be seen in FIGS. 5A and 5B, whereheat treatment causes the refractive indices of the first layer (FIG.5B) or second layer (FIG. 5A) (see the small dark dots) to revert totheir values in the cast web state (see the diamond-shaped symbols).Recall that the diamond-shaped symbols represent the refractive indicesof layers in the isotropic state (e.g., the cast web), the small darkdots represent the refractive indices of microlayers in the treated orselectively heated zones in the finished, internally patterned film, andthe open circles represent the refractive indices of microlayers inuntreated zones of the finished, internally patterned film.

If the birefringent material in the treated second zone is disorientedonly in part, i.e., incompletely, then the birefringent microlayersrelax to a state of birefringence that is less than the birefringentstate before heating but is not isotropic. In this case, the refractiveindices of the birefringent material in the treated second zone acquirevalues somewhere between the diamond-shaped symbols and the open circlesshown in FIGS. 5A-D. Some examples of such incomplete birefringentrelaxation are explained in more detail in commonly assigned PCTPublication WO 2010/075363 (Merrill et al.), “Internally PatternedMultilayer Optical Films With Multiple Birefringent Layers”,incorporated herein by reference.

In FIG. 5A, a first polymer material is selected that has a relativelylow refractive index, and a second polymer material is selected that hasa higher refractive index and that has a positive stress-opticcoefficient. The materials are coextruded in an alternating layerarrangement with a suitable number of layers to form a multilayer castweb, having indices shown by the diamond-shaped symbols. The cast web isthen uniaxially stretched along the x-axis under suitable conditions toinduce birefringence in the second polymer material while the firstpolymer material remains isotropic. The refractive index value n2 xincreases further to form a large index difference Δnx with n1 x. Therefractive index values n2 y and n2 z decrease to form small indexdifferences Δny and Δnz with n1 y and n1 z respectively. The values Δnyand Δnz may be zero, for example. This set of refractive indices, whenimplemented in a microlayer packet with an adequate number of layers,can provide a reflective polarizer with the x-axis being a block axisand the y-axis being a pass axis. The reflective polarizer may be broadband or narrow band, depending on the layer thickness distribution ofthe microlayers.

This reflective polarizing film can then be internally patterned in asecond zone as described above, while leaving the reflective polarizingfilm intact in a first zone. Selective heating by selective delivery ofradiant energy to the second zone causes the birefringent layers torelax to their original isotropic states, or to an intermediatebirefringent state if the disorientation is incomplete. If relaxation iscomplete, the second zone can become a mirror-like film (if themicrolayer packet has an adequate number of layers) with Δnx≈Δnyz≈Δnz.The finished film thus combines in a unitary film a reflective polarizerin one zone and a mirror-like film in a neighboring zone, withmicrolayers that extend continuously from one zone to the next. Suchfilms are described more fully in PCT Publication WO 2010/075340(Merrill et al.), “Multilayer Optical Films Having Side-by-SideMirror/Polarizer Zones”. For FIG. 5A, the selective heat treatmentprocess is able to change a multilayer reflective polarizer film to amultilayer reflective mirror film, i.e.: polarizer→mirror.

In FIG. 5B, first and second polymer materials are selected that havesubstantially the same refractive index, but where the first polymermaterial has a positive stress-optic coefficient. The materials arecoextruded in an alternating layer arrangement with a suitable number oflayers to form a multilayer cast web, having indices shown by thediamond-shaped symbols. The cast web is then biaxially stretched alongthe x- and y-axes under suitable conditions to induce birefringence inthe first polymer material while the second polymer material remainsisotropic. The refractive index values n1 x, n1 y increase to formsubstantial refractive index differences Δnx, Δny with n2 x, n2 yrespectively. The refractive index value n1 z decreases to form asubstantial refractive index difference Δnz with n2 z that is oppositein polarity or sign to Δnx and Δny. This set of refractive indices, whenimplemented in a microlayer packet with an adequate number of layers,can provide a mirror-like film. The reflection provided by the film maybe broad band or narrow band, depending on the layer thicknessdistribution of the microlayers.

This mirror-like film can then be internally patterned in a second zoneas described above, while leaving the mirror-like film intact in a firstzone. Selective heating by selective delivery of radiant energy to thesecond zone causes the birefringent layers to relax to their originalisotropic states, or to an intermediate birefringent state if thedisorientation is incomplete. If relaxation is complete, the second zonebecomes a window-like film with Δnxz≈Δnyz≈Δnz≈0. The finished film thuscombines in a unitary film a mirror-like reflector in one zone and asubstantial window in a neighboring zone, with microlayers that extendcontinuously from one zone to the next. For this FIG. 5B, the selectiveheat treatment process is able to change a multilayer reflective mirrorfilm to a multilayer window film (mirror→window).

In both FIGS. 5A and 5B, one of the optical materials remains isotropicafter stretching (and after the selective heat treatment). This,however, need not be the case in general, and many interesting anduseful multilayer optical film designs, as well as diffusely reflectivefilm designs, that can be converted into internally patterned opticalfilms using the selective heat treatment techniques disclosed hereincomprise two different optical materials for the constituent layers ofthe optical repeat unit, and both (rather than only one) of theseconstituent material layers become birefringent when the cast web isstretched or otherwise oriented. Such multilayer optical films anddiffusely reflective optical films are referred to herein as “doublybirefringent” optical films, since, in the case of multilayer opticalfilms, optical repeat units in such a film each include at least twoconstituent microlayers that are birefringent after stretching, and inthe case of diffusely reflective films, a blended layer in such a filmincludes at least two different materials that form two distinct phases,and both of the phases are birefringent after stretching.

When a doubly birefringent multilayer optical film is exposed to theselective heat treatment, a number of different responses are possiblein the treated zone depending on the material properties and the heatingconditions: both material layers may completely relax to becomeisotropic, or one material layer may relax completely or partially whilethe other material layer maintains its birefringence, or both materiallayers may relax by different amounts (e.g., one material layer mayrelax completely to become isotropic, while the other material relaxespartially so as to maintain only a portion of its birefringence), forexample. In any case, the change in birefringence of one or bothmaterial layers results in a reflective characteristic in the second(treated) zone of the optical film that differs substantially from areflective characteristic in the first (untreated) zone of the film.Further details of doubly birefringent multilayer optical films, andselective heating techniques used to internally pattern them, areprovided in the following commonly assigned PCT publications, which areincorporated herein by reference: WO 2010/075363 (Merrill et al.),“Internally Patterned Multilayer Optical Films With MultipleBirefringent Layers”; and WO 2010/075383 (Merrill et al.), “MultilayerOptical Films Having Side-by-Side Polarizer/Polarizer Zones”. Someexamples of doubly birefringent STOF films suitable for internalpatterning by selective heat treatment are shown in the presentapplication in FIGS. 5C and 5D.

In FIG. 5C, first and second polymer materials are selected that havethe same or similar isotropic refractive indices, and that have the sameor similar stress-optic coefficients (shown as positive in FIG. 5Calthough negative coefficients may also be used), and that havedifferent melting or softening temperatures. The materials arecoextruded in an alternating layer arrangement with a suitable number oflayers to form a multilayer cast web, having indices shown by thediamond-shaped symbols. Rather than being biaxially drawn, the cast webof FIG. 5C is then uniaxially stretched along the x-axis under suitableconditions to induce birefringence in both the first and second polymermaterials. The stretching causes the refractive index values n1 x and n2x to increase by similar amounts, while causing n1 z and n2 z todecrease by similar amounts, and while causing n1 y and n2 y to remainrelatively constant. This results in refractive indices of the twomaterial layers that are substantially matched along all three principaldirections (Δnx≈0, Δny≈0, and Δnz≈0), even though each material layer isstrongly biaxially birefringent. This set of refractive indices, whenimplemented in a microlayer packet with an adequate number of layers,can provide a multilayer window-like film that has little or noreflectivity for normally incident and obliquely incident light.

This multilayer window film can then be internally patterned in a secondzone as described above, while leaving the window film intact in a firstzone. Selective heating by selective delivery of radiant energy to thesecond zone causes at least some of the birefringent layers to relax,becoming less birefringent. In the case of FIG. 5C, the heating is againcarefully controlled to a temperature that is above the melting orsoftening point of the first material layers, but below the melting orsoftening point of the second material layers. In this way, theselective heating causes the first birefringent layers in the secondzone to relax to their original isotropic states, or to an intermediatebirefringent state if the disorientation is incomplete, while causingthe second birefringent layers in the second zone to substantiallymaintain their birefringence. If relaxation of the first material iscomplete, the second zone is characterized by a relatively largerefractive index difference (Δnx) in one in-plane direction, a zero ornear-zero refractive index difference (Δny) in the other in-planedirection, and a relatively large out-of-plane refractive indexdifference (Δnz) of opposite polarity or sign compared to Δnx. Theserefractive index relationships, when implemented in a microlayer packetwith an adequate number of layers, can provide a reflective polarizerfilm in the second zone. This polarizer film has a pass axis parallel tothe y-direction and a block axis parallel to the x-direction. Thereflection provided by this film for block-state polarized light may bebroad band or narrow band, depending on the layer thickness distributionof the microlayers. In either case, the reflectivity of the polarizerfilm for block-state polarized light (for both the s-polarized componentand the p-polarized component) increases with increasing incidence angledue to the opposite polarity of Δnz. The finished film thus combines ina unitary film a multilayer window film in one zone and a reflectivepolarizer film in a neighboring zone, with microlayers that extendcontinuously from one zone to the next. For this FIG. 5C, the selectiveheat treatment process is able to change a multilayer window film to amultilayer reflective polarizer film (window→polarizer).

The embodiment of FIG. 5D makes use of a two-step drawing process thatis described in U.S. Pat. No. 6,179,948 (Merrill et al.). In thisprocess, the stretching or orientation of the cast film is carried outusing a two-step drawing process that is carefully controlled so thatone set of layers (e.g., the first material layer of each optical repeatunit) substantially orients during both drawing steps, while the otherset of layers (e.g., the second material layer of each optical repeatunit) only substantially orients during one drawing step. The result isa multilayer optical film having one set of material layers that aresubstantially biaxially oriented after drawing, and having another setof material layers that are substantially uniaxially oriented afterdrawing. The differentiation is accomplished by leveraging the differentvisco-elastic and crystallization characteristics of the two materialsby using one or more suitably different process conditions such astemperature, strain rate, and strain extent for the two process drawingsteps. Thus, for example, a first drawing step may substantially orientthe first material along a first direction while at most only slightlyorienting the second material along this direction. After the firstdrawing step, one or more process conditions are suitable changed suchthat in a second drawing step, both the first and the second materialsare substantially oriented along a second direction. Through thismethod, the first material layers can assume an essentiallybiaxially-oriented character (for example, the refractive indices maysatisfy the relationship n1 x≈n1 y≠n1 z, sometimes referred to as auniaxially birefringent material), while the second material layers inthe very same multilayer film can assume an essentiallyuniaxially-oriented character (for example, the refractive indices maysatisfy the relationship n2 x≠n2 y≠n2 z≠n2 x, sometimes referred to as abiaxially birefringent material).

With this background, FIG. 5D depicts an embodiment in which the firstand second polymer materials are selected to have the same or similarisotropic refractive indices, and to both become birefringent afterdrawing, and to have the same polarity of stress-optic coefficient (inthe drawing they are both depicted as positive, but they can insteadboth be negative). The first and second materials have different meltingor softening temperatures, and have different visco-elastic and/orcrystallization characteristics such that the two-step drawing processdiscussed above can be implemented. The materials are coextruded in analternating layer arrangement with a suitable number of layers to form amultilayer cast web, having indices shown by the diamond-shaped symbols.The cast web is then biaxially stretched along the x- and y-axes usingthe above-described two-step drawing process, such that the firstmaterial is oriented comparably along both the x- and y-axes, whereasthe second material is oriented preferentially along the y-axis, withlesser orientation (including in some cases no orientation) along thex-axis. The net result is a multilayer optical film whose first andsecond microlayers are both birefringent, but the first material layershave a substantially biaxially-oriented character, whereas the secondmaterial layers have an asymmetric biaxially-oriented character, or evena substantially uniaxially-oriented character. As shown, the materialsand process conditions are selected so that the stretching causes therefractive index values n1 x and n1 y to increase by similar amounts,while causing n1 z to decrease by a larger amount. The stretching alsocauses the refractive index value n2 y to increase to a value equal toor close to that of n1 x and n1 y, and causes the refractive index n2 zto decrease, and causes the refractive index n2 x to remain about thesame (if the second material orients to a small degree during the x-axisorientation step, then n2 x may increase slightly as shown in thefigure). This results in refractive indices of the two material layersthat have one large in-plane refractive index mismatch (Δnx), onesignificantly smaller in-plane refractive index mismatch (Δny≠0), and anintermediate out-of-plane refractive index mismatch (Δnz) of oppositepolarity from Δnx. When the second material orients more biaxially,index matching in the x-direction after treatment may be achieved bypairing with a first material whose isotropic index is higher than thesecond. This set of refractive indices, when implemented in a microlayerpacket with an adequate number of layers, can provide a first reflectivepolarizing film with a block axis along the x-direction and a pass axisalong the y-direction. The reflection provided by the film (for lightpolarized parallel to the block axis) may be broad band or narrow band,depending on the layer thickness distribution of the microlayers.

This first multilayer reflective polarizer film can then be internallypatterned in a second zone as described above, while leaving thepolarizer film intact in a first zone. Selective heating by selectivedelivery of radiant energy to the second zone causes at least some ofthe birefringent layers to relax, becoming less birefringent. In thepresent case, the heating is carefully controlled to a temperature thatis above the melting or softening point of the first material layers,but below the melting or softening point of the second material layers.In this way, the selective heating causes the first birefringent layersin the second zone to relax to their original isotropic states, or to anintermediate birefringent state if the disorientation is incomplete,while causing the second birefringent layers in the second zone tosubstantially maintain their birefringence. If relaxation of the firstmaterial is complete, the second zone is characterized by a relativelylarge refractive index difference (Δny) in one in-plane direction, azero or near-zero refractive index difference (Δnx) in the otherin-plane direction, and a relatively large out-of-plane refractive indexdifference (Δnz) of opposite polarity or sign compared to Δny. Theserefractive index relationships, when implemented in a microlayer packetwith an adequate number of layers, can provide a second reflectivepolarizer film in the second zone. Notably, this second polarizer has apass axis parallel to the x-direction and a block axis parallel to they-direction, i.e., it is perpendicularly oriented relative to the firstreflective polarizer. The reflection provided by this second polarizerfilm for block-state polarized light will be broad band or narrow band,depending on the layer thickness distribution of the microlayers, to thesame extent that the first reflective polarizer is broad band or narrowband for the orthogonal polarization state. In any case, thereflectivity of the second polarizer film for block-state polarizedlight (for both the s-polarized component and the p-polarized component)increases with increasing incidence angle due to the opposite polarityof Δnz in the second zone. The finished film thus combines in a unitaryfilm a first reflective polarizer film in one zone and a secondreflective polarizer film in a neighboring zone, the second reflectivepolarizer film being oriented perpendicular to the first reflectivepolarizer film, with microlayers that extend continuously from one zoneto the next. For this FIG. 5D, the selective heat treatment process isable to change a first multilayer reflective polarizer film to a secondmultilayer reflective polarizer film (polarizer1→polarizer2).

The scenarios discussed above involve only some of a multitude ofpossible combinations of reflector types for the first zone, reflectortypes for the second zone, material characteristics, and processingparameters that can be used to produce other internally patternedmultilayer optical films, and should not be considered to be limiting.Note that not just positively birefringent but also negativelybirefringent materials, and combinations thereof, can be used. Note alsothat in cases where the combination of a birefringent and isotropicpolymer is used, the birefringent polymer may have a pre-stretchisotropic index that is less than, greater than, or equal to therefractive index of the isotropic polymer. Discussion of other possiblecombinations of reflector types for the first and second zones ofinternally patterned multilayer optical films, which variouscombinations can be utilized in bi-level writeable multilayer opticalfilms as disclosed herein, can be found in one or more of the followingcommonly assigned PCT publications: WO 2010/075357 (Merrill et al.),“Internally Patterned Multilayer Optical Films Using Spatially SelectiveBirefringence Reduction”; WO 2010/075340 (Merrill et al.), “MultilayerOptical Films Having Side-by-Side Mirror/Polarizer Zones”; WO2010/075363 (Merrill et al.), “Internally Patterned Multilayer OpticalFilms With Multiple Birefringent Layers”; and WO 2010/075383 (Merrill etal.), “Multilayer Optical Films Having Side-by-Side Polarizer/PolarizerZones”.

FIG. 6 is a schematic diagram that summarizes various transformationsthat can be achieved using the birefringent-relaxation techniquesdiscussed herein for multilayer optical films. As such, the diagram alsosummarizes a variety of combinations of reflector types for the first(untreated) zone and the second (heat treated) zone of an internallypatterned multilayer optical film, which in turn may form part of abi-level writeable composite film, which may also comprise one or morepatternable retarder films. The arrows in the figure representtransformations from a first reflective characteristic to a secondreflective characteristic that differs substantially from the firstreflective characteristic. Note that the diagram of FIG. 6 is providedfor illustrative purposes and should not be construed as limiting.

Arrow 610 a represents a transformation from a multilayer mirror film toa multilayer window film, e.g., as described in connection with FIG. 5B.Such a transformation can be used to provide an internally patternedmultilayer optical film with one or more first (untreated) zonescharacterized by a mirror film and one or more second (treated) zonescharacterized by a window film. Arrow 610 b represents an oppositetransformation, from a multilayer window film to a multilayer mirrorfilm. Such a transformation can be used to provide an internallypatterned multilayer optical film with one or more first (untreated)zones characterized by a window film and one or more second (treated)zones characterized by a mirror film.

Arrow 612 a represents a transformation from a multilayer window film toa multilayer polarizer film, e.g., as described in connection with FIG.5C. Such a transformation can be used to provide an internally patternedmultilayer optical film with one or more first (untreated) zonescharacterized by a window film and one or more second (treated) zonescharacterized by a polarizer film. Arrow 612 b represents an oppositetransformation, from a multilayer polarizer film to a multilayer windowfilm. Such a transformation can be used to provide an internallypatterned multilayer optical film with one or more first (untreated)zones characterized by a polarizer film and one or more second (treated)zones characterized by a window film.

Arrow 614 a represents a transformation from a multilayer polarizer filmto a multilayer mirror film, e.g., as described in connection with FIG.5A. Such a transformation can be used to provide an internally patternedmultilayer optical film with one or more first (untreated) zonescharacterized by a polarizer film and one or more second (treated) zonescharacterized by a mirror film. Arrow 614 b represents an oppositetransformation, from a multilayer mirror film to a multilayer polarizerfilm. Such a transformation can be used to provide an internallypatterned multilayer optical film with one or more first (untreated)zones characterized by a polarizer film and one or more second (treated)zones characterized by a window film.

Arrows 616, 618, and 620 represent transformations from one type ofmirror to another type of mirror, from one type of window to anothertype of window, and from one type of polarizer to another type ofpolarizer (see e.g. FIG. 5D). The reader is again reminded that thediagram of FIG. 6 is provided for illustrative purposes and should notbe construed in a limiting fashion.

FIGS. 5A-D and 6 and their associated descriptions are primarilydirected to reflective films whose reflective characteristics aredetermined in large part by constructive and destructive interference oflight reflected from interfaces between microlayers disposed within thefilm, i.e., multilayer optical films. Counterparts to those figures anddescriptions can also be provided for reflective films whose reflectivecharacteristics are diffuse in nature because they are determined inlarge part by first and second materials that are separated intodistinct first and second phases in one or more blended layer. Referencein this regard is made to commonly assigned U.S. Application Ser. No.61/360,124, “Diffuse Reflective Optical Films With Spatially SelectiveBirefringence Reduction”, filed Jun. 30, 2010. For each of FIGS. 5A-D,the “first” material may be considered to be a continuous phase and the“second” material may be considered to be a dispersed phase (or anothercontinuous phase), while in an alternative embodiment, the “second”material may be considered to be a continuous phase and the “first”material may be considered to be a dispersed phase (or anothercontinuous phase).

The fact that the change in the reflective characteristic of the STOFfilm is associated primarily with heat-induced relaxation inbirefringence of a material or layer of the STOF film means that theselective treatment process used to pattern the STOF film may beprimarily one-way or irreversible. For example, a given area or zone ofthe STOF film that has been processed (selectively heat treated byabsorption of radiant energy) so that its initial first reflectioncharacteristic has been changed to a second reflection characteristicmay thereafter not be able to be processed with another radiant beam tore-acquire its original first reflection characteristic. In fact, if theinitial heat treatment substantially eliminated birefringence in thezone, then further radiant treatment with the same or similar radiantbeam may have little or no additional effect on the reflectivecharacteristic of the zone. This one-way or irreversible aspect of STOFfilm patterning may be particularly advantageous e.g. in securityapplications where, for example, tamper-resistance is important, or indisplay or opto-electronic applications where for example stability tooptical or electronic fields, used to switch other component elements,is desired. In other applications, this one-way or irreversible aspectof STOF film patterning in a continuous phase may be combined withswitchable elements in another phase, e.g. in opto-electronic deviceswhere for example a stable, patterned continuous phase withbirefringence in a first zone and little or no birefringence in a secondzone is desired.

In FIG. 7, we show one arrangement 700 that can be used to selectivelyheat the second zone of a STOF film to provide the disclosed patterned(e.g. internally patterned) films. Briefly, a STOF film 710 is providedthat comprises at least one patternable reflective film that extendsthroughout the film, e.g. from a first to a second zone. The reflectivefilm may be internal to the STOF film, and provides a first reflectivecharacteristic. A high radiance light source 720 provides a directedbeam 722 of suitable wavelength, intensity, and beam size to selectivelyheat an illuminated portion 724 of the film by converting some of theincident light to heat by absorption. Preferably, the absorption of thefilm is great enough to provide sufficient heating with areasonably-powered light source, but not so high that an excessiveamount of light is absorbed at the initial surface of the film, whichmay cause surface damage. This is discussed further below. In some casesit may be desirable to orient the light source at an oblique angle θ, asshown by the obliquely positioned light source 720 a, directed beam 722a, and illuminated portion 724 a. Such oblique illumination may bedesirable where the STOF film 710 contains a microlayer packet having areflection band at normal incidence that substantially reflects thedirected beam 722 in a way that prevents the desired amount ofabsorption and concomitant heating. Thus, taking advantage of the shiftof the reflection band to shorter wavelengths with increasing incidenceangle, the directed beam 722 a can be delivered at an oblique angle θthat avoids the (now shifted) reflection band to allow the desiredabsorption and heating.

Oblique illumination may also be desirable where the STOF film 710includes a diffuse reflector, and were the diffuse reflectivity changeswith incidence angle and/or polarization state. For asymmetric diffusereflectors, like reflective polarizers, it may also be desirable toorient the light source at a controlled azimuthal angle 4. At oneincident direction (defined e.g. by a given (θ,φ) coordinate pair) andpolarization state, for example, the film may scatter the directed beam722/722 a to a great extent in a way that prevents the desired amount ofabsorption and concomitant heating of the blended layer in the secondzone. At a different incident direction (θ,φ) and/or polarization state,the scattering may be substantially reduced so as to allow the desiredamount of absorption and concomitant heating of the blended layer in thesecond zone to produce the birefringence relaxation and reflectivitytransformations discussed above. Thus, the incident direction (θ,φ) andthe polarization state of the directed beam 722/722 a can be selected toavoid excessive scattering through the blended layer, e.g., they can beselected to coincide with a minimum scattering of the blended layer oroptical film, or stated differently to coincide with a maximum ofspecular transmission through the blended layer. If the diffuselyreflective film is a reflective polarizer, the polarization state maydesirably be a pass state of the polarizer.

In some cases, the directed beam 722 or 722 a may be shaped in such away that the illuminated portion 724 or 724 a has the desired shape ofthe finished second zone. A mask may be used for that purpose. In othercases, the directed beam may have a shape that is smaller in size thanthe desired second zone. In the latter situation, beam steeringequipment can be used to scan the directed beam over the surface of themultilayer optical film so as to trace out the desired shape of the zoneto be treated. Spatial and temporal modulation of the directed beam canalso be utilized with devices such as beam splitters, lens arrays,pockels cells, acousto-optic modulators, and other techniques anddevices known to those of ordinary skill in the art.

FIGS. 8A-C provide schematic top views of different second zones of apatterned STOF film, and superimposed thereon possible paths of adirected light beam relative to the film capable of forming the depictedzones. In FIG. 8A, a light beam is directed at a STOF film 810 andscanned at a controlled speed from a starting point 816 a to an endingpoint 816 b along a path 816 to selectively heat the film in anarbitrarily-shaped zone 814 to distinguish it from a first zone 812.FIGS. 8B and 8C are similar. In FIG. 8B, a light beam is directed at aSTOF film 820 and scanned at a controlled speed from a starting point826 a along a path 826 to selectively heat the film in arectangular-shaped zone 824 to distinguish it from a neighboring firstzone 822. In FIG. 8C, a light beam is directed at a STOF film 830 andscanned at controlled speeds along the discontinuous paths 836-842, andso on, to selectively heat the film in a rectangular-shaped zone 834 todistinguish it from a neighboring first zone 832. In each of FIGS. 8A-C,the heating is sufficient to reduce or eliminate birefringence of atleast some interior layers or materials in the second zone whilemaintaining the birefringence of those layers or materials in the firstzone, and is accomplished while maintaining the structural integrity ofthe layers or films in the second zone and without any selectiveapplication of pressure to the second zone.

The directed light beam may also be modulated to create paths that aredashed, dotted, or otherwise broken or discontinuous. The modulation maybe complete, wherein the light beam intensity changes from 100% or “fullon” to 0% or “full off”. Alternatively, the modulation may be partial.Further, the modulation may include abrupt (e.g. stepwise) changes inbeam intensity, and/or it may include more gradual changes in beamintensity.

FIGS. 9A and 9B address the topic of how the absorption of thepatternable film can or should be tailored to provide optimal localizedheating. The graphs of FIGS. 9A and 9B are plotted on the samehorizontal scale, which represents the depth or position of the radiantlight beam as it propagates through the film. A depth of 0% correspondsto the front surface of the film, and a depth of 100% corresponds to therear surface of the film. FIG. 9A plots along the vertical axis therelative intensity I/I₀ of the radiant beam. FIG. 9B plots the localabsorption coefficient (at the selected wavelength or wavelength band ofthe radiant beam) at each depth within the film.

Three curves are plotted in each figure, for three patternable STOF filmembodiments. In a first embodiment, the film has a substantially uniformand low absorption throughout its thickness at the wavelength of thedirected light beam. This embodiment is plotted as curve 910 in FIG. 9Aand curve 920 in FIG. 9B. In a second embodiment, the film has asubstantially uniform and high absorption throughout its thickness. Thisembodiment is plotted as curve 912 in FIG. 9A and curve 922 in FIG. 9B.In a third embodiment, the film has a relatively low absorptionthroughout regions 915 a and 915 c of its thickness, but has a higher,intermediate absorption in region 915 b of its thickness.

The first embodiment has an absorption coefficient that is too low formany situations. Although the directed light beam is absorbed uniformlyas a function of depth as indicated by the constant slope of the curve910, which may be desirable in some cases, very little of the light isactually absorbed as shown by the high value of the curve 910 at a depthof 100%, meaning that a high percentage of the directed light beam iswasted. Nevertheless, in some cases this first embodiment may still bequite useful in the treatment of some films. The second embodiment hasan absorption coefficient that is too high for many situations. Althoughsubstantially all of the directed light beam is absorbed, and none iswasted, the high absorption causes an excessive amount of light to beabsorbed at the front surface of the film, which may cause surfacedamage to the film. If the absorption is too high, an adequate amount ofheat cannot be delivered to interior layers or materials of interestwithout damaging layers at or near the front surface of the film. Thethird embodiment utilizes a non-uniform absorption profile that may beachieved, for example, by incorporating an absorbing agent into selectedinterior layers of the film. The level of absorptivity (controlled bythe local absorption coefficient) is desirably set to an intermediatelevel so that an adequate portion of the directed light beam is absorbedin the tailored absorbing region 915 b of the film, but the absorptivityis not so high that an excessive amount of heat is delivered to theincident end of the region 915 b relative to the opposite end. In manyinstances, the absorptivity in absorbing region 915 b is stillreasonably weak, e.g. the relative intensity profile 914 over thatregion may appear more as a straight line with merely a steeper slopethan the other regions, e.g. 915 a and 915 c. The adequacy of theabsorption may be determined by balancing that absorptivity against thepower and duration of the incoming directed light beam to achieve thedesired effect.

In an elementary example of the third embodiment, the patternable STOFfilm may have a construction of two thick skin layers with one or morepackets of microlayers therebetween (separated by protective boundarylayers if two or more microlayer packets are included), and the film maybe composed of only two polymer materials A and B. An absorbing agent isincorporated into polymer material A to increase its absorptivity to amoderate level but no absorbing agent is incorporated into polymer B.Both materials A and B are provided in alternating layers of themicrolayer packet(s), but the skin layers and the protective boundarylayers, if present, are composed only of polymer B. Such a constructionwill have a low absorptivity at the outer surfaces, i.e. the skinlayers, of the film, due to the use of the weakly absorbing material B,and will also have a low absorptivity at the optically thick PBLs ifthey are present. The construction will have a higher absorptivity inthe microlayer packet(s) due to the use of the more strongly absorbingmaterial A in alternating microlayers (along with alternatingmicrolayers of the more weakly absorbing material B). Such anarrangement can be used to preferentially deliver heat to interiorlayers of the film, e.g. to one or more interior microlayer packet(s),rather than to outer surface layers. Note that with an appropriatelydesigned feedblock the multilayer optical film can comprise three ormore different types of polymer materials (A, B, C, . . . ), and anabsorptive agent may be incorporated into one, some, or all of thematerials in order to provide a wide variety of different absorptionprofiles so as to deliver heat to selected interior layers, packets, orregions of the film. In other cases, it may be useful to include anabsorbing agent in the PBL(s) or even in the skin layer, if present. Ineither case, the loading or concentration may be same or different,either higher or lower, than in the microlayers.

Similar absorption profiles as those of the foregoing embodiments may beobtained using the inherent absorption characteristics of the variousnative materials used in the multilayer optical film. Thus, a givencomposite film construction may comprise different materials havingdifferent absorption characteristics among the various constituentlayers or films of the composite film, and those various layers or filmsmay have been formed together during film formation (e.g. bycoextrusion), or may have been formed as separate precursor films whichwere later combined e.g. by lamination.

Distinctive articles with unique capabilities can be obtained bycombining one or more STOF films with one or more masks in a compositearticle. The STOF film(s) may be any of the wide variety of STOF filmsdisclosed herein. The mask(s) may be of any conventional design, but mayalso include structured surface features that may be adapted topreferentially redirect light from a radiant beam onto selected portionsof the STOF film(s) as discussed further below. The STOF film(s) may becombined with the mask(s) by attachment, e.g., attachment by directlamination, or using a transparent adhesive layer and/or otherattachment layers. Attachment is in a generally layered arrangement suchthat at least some light that passes through the mask impinges upon thereflective STOF film.

A variety of structures on the surface of a mask, or embedded within amask, may be used to manipulate the local flux density of the appliedradiant energy of treatment over the layers or packets of interest inthe STOF film. The structures may be regular or variable in shape andperiodic or aperiodic (quasi-random or chaotic) in arrangement. Thestructures may have one dimensional or two dimensional qualities acrossthe plane of the film, i.e., a given structure may have across-sectional shape that is uniform and unchanging along one in-planeaxis of the film (e.g. in the case of a linearly extending prism), or agiven structure may be bounded in two perpendicular in-plane directionswith two cross-sectional shapes (e.g. in the case of a hemisphericalprotrusion or pyramidal structure). The structures may fully or onlypartially cover the surface, e.g. flat regions may intervene betweenisolated structures, clusters of structures, or interweave across thesurface. The structures may alter the relative intensities or fluxes ofan applied radiant beam as a function of in-plane coordinates. In somecases, the structures may also affect the relative intensities or fluxesthrough the thickness of the STOF film. Various mechanisms include butare not limited to compression (or conversely, rarefication) of the fluxfrom an initially incident planar projection onto the structures to aneffective planar projection onto the STOF film itself, overlapping areasof the initial incident regions onto the STOF film, and filtering of thetransmission into the structures as a result of the dependence of thereflection coefficient on incidence angle and polarization, as well asthe material considerations of the region above the structures and thestructures themselves, most notably the indices of refraction and theirbulk and surface haze. The manipulations may also depend upon the levelof collimation of the incident light. For example, the incident lightmay be normally incident and collimated, or may be partially collimatede.g. using a collimating film or films (such as Vikuiti™ BrightnessEnhancement Films (BEF) available from 3M Company, St. Paul, Minn.,whether used alone, or with two such films crossed with each other, orcombined with other materials), or may be uncollimated as in the case ofa Lambertian source. A polarizer may also be used to pre-polarize theincident light from the light source. In this manner, the variousportions of the STOF film may experience different amounts of flux, anddifferent levels of treatment. For example, areas of higher intensity orflux may become treated (i.e., when exposed to a radiant beam, heated toan extent sufficient to reduce the birefringence of one or morematerials in the STOF film) and represent portions of a second (treated)zone of a patterned STOF film, while areas of lower intensity or fluxmay remain untreated or less treated (i.e., when exposed to a radiantbeam, heated to an extent that is not sufficient to substantially reducethe birefringence of one or more materials in the STOF film) andrepresent portions of a first (untreated) zone of the patterned STOFfilm.

The structures on the surface of the mask may have characteristicdimensions that manipulate light chiefly by the phenomenon ofrefraction, or they may have smaller dimensions so as to manipulatelight chiefly by the phenomenon of diffraction, or they may be sized tomanipulate light by a combination of refraction and diffraction.

Further with regard to composite articles that include one or more STOFfilms with one or more masks, FIGS. 10-12 provide schematic side orsectional views of various articles that each include a STOF reflectivefilm attached to a mask having a structured surface. In each of thesefigures, the shape of the structures is shown in only one plane, i.e.,the plane of the drawings, which is referred to as an x-z plane. In theorthogonal direction (i.e., along the y-axis), the structures may extenduniformly, as in the case of linearly extending prisms, or they may havea bounded profile, for example, the same as or similar to theirrespective profiles in the x-z plane. Structures that areone-dimensional in character, e.g., maintain a consistentcross-sectional profile along the y-direction, may represent linearcylindrical structures, such as curved lenticular or flat-facetedtriangular prismatic structures. Structures that are two-dimensional incharacter, i.e., have a bounded profile in both the x-z and y-z planes,or that have a bounded profile in one plane and the bounded profile isnon-uniform along an orthogonal direction, may function as lens-likeprotuberances.

Briefly, FIG. 10 depicts a composite article 1010 that includes a mask1012 attached to a STOF film 1014. The mask 1012 may be lighttransmissive (transparent) over its entire useable area. The mask 1012has a structured surface by virtue of the curved structures 1012 a, 1012b on an otherwise flat upper surface. The structures 1012 a, 1012 b areshown to increase the flux of a radiant beam 1016 in corresponding areasor zones 1014 a, 1014 b respectively, which zones may then be treated(reflection characteristic modified by the mechanism of birefringencereduction) by the radiant beam 1016 while other zones of the STOF film1014 are not. FIG. 11 depicts a composite article 1110 that includes atransparent mask 1112 attached to a STOF film 1114. The mask 1112 has astructured surface characterized by curved (e.g. semicircular orhemispherical) structures 1112 a, which have a transverse dimension 1113a. The mask 1112 may have a “land” portion of dimension (thickness) 1113b. The STOF film may have a thickness of 1115 a. FIG. 12 depicts acomposite article 1210 that includes a transparent mask 1212 attached toa STOF film 1214 and to another optical film or substrate 1216. The mask1212 has a structured surface characterized by triangular prismaticstructures 1212 a, which have a transverse dimension 1213 a. The mask1212 may have a “land” portion of dimension (thickness) 1213 b. The STOFfilm may have a thickness of 1215 a. The mask 1012 of FIG. 10 is alsoshown as having a finite land portion, which is not specificallylabeled.

Composite articles such as those of FIGS. 10-12 may function as lightdirecting films, e.g., where treated portions of the STOF film are lessreflective and more transmissive than untreated areas. Such a compositearticle, if used in a sign, backlight, luminaire, or similar internallylit device, may be oriented so that the STOF film is disposed towardsthe internal light source and the structured surface mask is disposedaway from such light source. Treated areas of the STOF film may thentransmit light, which may then be collimated or partially collimated, orotherwise directed along a desired output axis, by virtue of the spatialregistration of the structured surface features with the treated areasof the STOF film.

The surface structure or profile of the depicted masks may becharacterized by a surface function f(x,y) above a land of thickness H.In FIG. 11, H is labeled 1113 b, and in FIG. 12, H is labeled 1213 b. Insome cases, the land may be omitted (H=0), such that the structuresattach directly to the STOF film. In some cases, the land may be anouter skin layer of the STOF film, and the structures may be formeddirectly on the skin layer, e.g. via any number of surface structuringmethods including extrusion replication, embossing, and surfacemachining (e.g. diamond turning). When the structure has one-dimensionalcharacter, the surface structure may be defined by a surface function ofonly one in-plane direction, e.g. f(x). The structures need not bealigned with the any of the principal directions of the dielectrictensor of the STOF film at an optical band of interest, although theymay be in some cases.

In some cases, an additional upper layer (made of a transparent materialdenser than air) may be provided that completely covers the structuredsurface of the mask and encapsulates the individual structures. Such anupper layer may be composed of a transparent “upper” medium, having arefractive index tensor n_(U). The upper medium may be isotropic. Themasks 1012, 1112, 1212, including their respective structures andoptional land, may comprise a “lower” medium or material with refractiveindex tensor n_(L). The lower medium may be isotropic or may bebirefringent, e.g. as described in Patent Application Publications US2007/0065636 (Merrill et al.) and US 2006/0204720 (Biernath et al.). Δnygiven structure may have a basal width B (labeled 1113 a in FIGS. 11 and1213 a in FIG. 12) along a given direction and a local surface normalvector at each planar projection position in the (x,y) coordinate plane.In FIG. 11, light that is normally incident (parallel to the z-axis)over a differential cross-section width Δx (Δx<<B), impinges on thesurface of one of the individual structures. Some portion of thisincident light is reflected in accord with the reflection coefficientsgiven the local surface normal, the refractive indices n_(U) and n_(L),and the polarization state of the incident radiation. The transmittedflux over the width Δx is then transmitted to the surface of the STOFfilm over a new cross sectional width Δx′. In FIG. 11, the new crosssection Δx′ is smaller, thus the intensity is increased in thisillustrative example. In FIG. 12, two initially incident planarprojections onto the structures at Δx₁ and Δx₂ are directed to overlaponto the STOF film at a single effective planar projection Δx′.Compression and overlap can be combined to further increase theintensity over selected portions of the STOF film.

The relative sizes and/or heights of the surface function f(x,y), baseB(x,y), H, and the thickness of the STOF film can be selected to achievedesirable effects. For example, it may be advantageous for thestructures to be large relative to the thickness of the optical layer tobe treated in the STOF film, in order to achieve a uniform controlledpattern of modified reflection characteristics. It may also beadvantageous to adjust H to achieve a certain level of focus, or toposition a focal plane, e.g. above, below, or within the STOF film. Itmay be advantageous to use fine structures, e.g. where B and/or H are ofthe order of the thickness of the STOF film so that the structured maskeffectively controls the areal conversion or treatment, e.g. via ahalf-tone approach, of the STOF film, e.g. to achieve a controlled levelof change in the reflective characteristics such as control of light ina security, decorative, display, sign, or lighting system.

In one example, a structured mask is formed to coincide with a patternedarray in an optical device. For example, the mask may be patterned toalign with light sources, e.g. bulbs in an LED direct-backlit display.After treatment, a pattern of desired consonance with the array of lightsources in the optical device can be formed. The patterned STOF film maybe thus patterned and registered with the components of the opticaldevice in a second step. Alternatively, the patterned mask may itself bea patterned optical feature of the optical device. Treatment through thepatterned mask in the assembled or sub-assembled optical part may thencreate a self-registered patterning of the STOF film and this otheroptical feature.

Thus, for example, a STOF film may be patterned to have first zones(whether untreated or treated) of low reflectivity and hightransmission, and second zones (whether treated or untreated) of highreflectivity and low transmission, where the pattern is tailored tomatch a pattern of LED or other light sources of a backlight useful in adisplay, or in a luminaire. The highly reflective second zones may bepositioned above or in front of the light sources, respectively, toprevent light from the light sources from directly reaching theobserver's eye or from otherwise producing “hot spots”, i.e., locallybright areas. Light reflected by the patterned STOF film may bereflected by a back reflector and may pass through the highlytransmissive first zones of the STOF film, towards a front of thebacklight. In this regard, the backlight may be or comprise a recyclingbacklight.

The STOF films and articles may thus be used in a wide variety ofdisplays and other extended area optoelectronic devices, such asbacklights, signs, luminaires, channel letters, light guiding or lightpiping systems, and the like. Such devices may emit polarized orunpolarized light. Such devices may emit white light, i.e., lightperceived by an ordinary observer as nominally white, or light of aparticular color other than white. Such devices may comprise arrays ofliquid crystals, organic light emitting devices (OLEDs), and/or lightemitting diodes (LEDs), for example. Such devices may be or comprise a3-dimensional display, e.g., a stereoscopic display. Such devices may beor comprise transmissive displays, reflective displays, and/ortransflective displays. Such devices may include edge-lit displaysand/or direct-lit displays.

The films, methods, and business processes disclosed herein may begenerally useful in any application in which a spatially controlledlevel of orientation is desired. Fields of interest may include, forexample, display, decorative, and security applications. Someapplications may overlap multiple fields. For example, some articles mayincorporate the internally patterned films disclosed herein incombination with a film, substrate, or other layer that includesconventional patterning in the form of indicia, for example. Theresulting article may be useful in security applications, but versionsof it may also be considered decorative. Selectively heat treating suchan article may produce zones in the internally patterned film thatselectively obstruct (by increasing reflectivity) or reveal (bydecreasing reflectivity) portions of the conventional patterning of theother film, depending on the design of the internally patterned film.Color shifting characteristics of the disclosed internally patternedfilms may also be exploited in combination with colored orblack-and-white background indicia as disclosed for example in U.S. Pat.No. 6,045,894 (Jonza et al.) “Clear to Colored Security Film”, and U.S.Pat. No. 6,531,230 (Weber et al.) “Color Shifting Film”.

Further in regard to security applications, the disclosed films may beused in a variety of security constructions including identificationcards, driver's licenses, passports, access control passes, financialtransaction cards (credit, debit, pre-pay, or other), brand protectionor identification labels, and the like. The film may be laminated orotherwise adhered as interior or exterior layers to other layers orportions of the security construction. When the film is included as apatch, it may cover only a portion of the major surface of the card,page, or label. In some cases, it may be possible to use the film as thebase substrate or the only element of the security construction. Thefilm may be included as one of many features in the securityconstruction such as holograms, printed images (intaglio, offset,barcode, etc.), retroreflective features, UV or IR activated images andthe like. In some instances, the disclosed films may be layered incombination with these other security features. The film may be used toprovide a personalizable feature to the security construction, forexample, a signature, an image, an individual coded number, etc. Thepersonalizable feature may be in reference to the individual documentholder or a specific product entity, such as in the case of amanufacturer tag, a lot verification tag, a tamper-proof coding, or thelike. The personalizable feature can be made with a variety of scanningpatterns including lines and dot patterns. Patterns can be the same ordifferent among writable packets, depending on the film construction.

Consider, for example, the case of a first writable packet ofmicrolayers that initially exhibits a perceptible color but then becomesclear upon treatment or patterning. One or more such color packets canbe used. Consider the addition of a second multilayer optical filmpacket to form the film construction to be included in the securityconstruction. Patterning or writing the first packet will create adesign or image in the color of the second packet in a backgroundrepresenting the color characteristics of the two packets combined. Whenthe spectral bands are sufficiently narrow, both the foreground(patterned area) and background can color shift with viewing angle. Thevariation of the perceived color with background, e.g. white or blackbackgrounds, to favor transmitted or reflected light viewing can be usedas a security feature. For example, a page or leaf of the film in adocument, such as a passport, can be flipped to view the film againstdifferent backgrounds or portions of the document.

The STOF films may provide both overt (e.g. clearly visible to anordinary observer) and covert security features to the securityconstruction. For example, a writable (color) reflective polarizer layercan provide a covert feature viewable with a polarizing analyzer, e.g. afeature that changes color or disappears depending on the polarizationstate of the analyzer. An infrared reflecting packet can be patterned tomake an IR detectable, e.g. machine readable, personalized codingfeature.

A particularly interesting STOF film construction for securityapplications is a very far red or near IR reflector, e.g., with a lower(left) reflection band edge between 650 and 800 nm (depending on filmconstruction) as described in U.S. Pat. No. 6,045,894 (Jonza et al.),which can provide a clear-to-colored appearance as the observation anglechanges from normal incidence to glancing incidence. Other interestingconstructions, including optical polarizing films with designed colorshifts, are described in U.S. Pat. No. 7,064,897 (Hebrink et al.). Usingthe patterning methods of the present application, films such as thosedescribed in the '894 Jonza reference and those described in the '897Hebrink reference can be made that are writable, for example, with alaser. For example, personalized information can be written into such afilm by the alteration of reflection packets in the visible, UV, or IRportion of the spectrum, where the altered portions (treated zones) ofthe film may have a lower reflectivity than untreated portions of thefilm, or vice versa.

Additional useful articles that can be made using the disclosed STOFfilms include a wide variety of identification documents (ID documents).The term “ID documents” is broadly defined and is intended to include,but not be limited to, passports, driver's licenses, national ID cards,social security cards, voter registration and/or identification cards,birth certificates, police ID cards, border crossing cards, securityclearance badges, security cards, visas, immigration documentation andcards, gun permits, membership cards, phone cards, stored value cards,employee badges, debit cards, credit cards, and gift certificates andcards. ID documents are also sometimes referred to as “securitydocuments”. The articles of this disclosure may be the ID document ormay be part of the ID document. Other useful articles that may be madeusing the disclosed patternable films include articles containing colorimages and items of value, such as, for example, currency, bank notes,checks, and stock certificates, where authenticity of the item isimportant to protect against counterfeiting or fraud, as well asarticles that can be used to produce informative, decorative, orrecognizable marks or indicia on product tags, product packaging,labels, charts, maps, and the like.

Still more useful articles that can utilize the disclosed STOF filmsinclude passports, ID badges, event passes, affinity cards, productidentification formats and advertising promotions for verification andauthenticity, brand enhancement images, identification presentationimages in graphics applications such as emblems for police, fire, orother emergency vehicles; information presentation images in graphicsapplications such as kiosks, night signs, and automotive dashboarddisplays; and novelty enhancement through the use of composite images onproducts such as business cards, hang-tags, art, shoes, and bottledproducts.

Finally, it should be noted that many of the features described here forsecurity applications are likewise useful for decorative applications.For example, a personalized logo can be thus embedded in a consumerarticle.

The teachings of this application can be used in combination with theteachings of any or all of the following commonly assigned applications,which are incorporated herein by reference: PCT Publication WO2010/075357 (Merrill et al.), “Internally Patterned Multilayer OpticalFilms Using Spatially Selective Birefringence Reduction”; PCTPublication WO 2010/075340 (Merrill et al.), “Multilayer Optical FilmsHaving Side-by-Side Mirror/Polarizer Zones”; PCT Publication WO2010/075373 (Merrill et al.), “Multilayer Optical Films Suitable forBi-Level Internal Patterning”; PCT Publication WO 2010/075363 (Merrillet al.), “Internally Patterned Multilayer Optical Films With MultipleBirefringent Layers”; and PCT Publication WO 2010/075383 (Merrill etal.), “Multilayer Optical Films Having Side-by-Side Polarizer/PolarizerZones”; and the following applications filed on Jun. 30, 2010: U.S.Application Ser. No. 61/360,124, “Diffuse Reflective Optical Films WithSpatially Selective Birefringence Reduction”; U.S. Application Ser. No.61/360,127, “Retarder Film Combinations With Spatially SelectiveBirefringence Reduction”; U.S. Application Ser. No. 61/360,022,“Multi-Layer Articles Capable of Forming Color Images and Methods ofForming Color Images”; and U.S. Application Ser. No. 61/360,032,“Multi-Layer Articles Capable of Forming Color Images and Methods ofForming Color Images”.

In many cases, a material layer or phase will exhibit birefringence as aresult of the molecular makeup of the material. In some cases, however,a medium (sometimes referred to as an effective medium) may exhibitbirefringence as a result of microscopic structures that have adimension that is small compared to the wavelength of light but largecompared to molecular distances. An elementary example of such a mediumis a stack of ultrathin layers of different light-transmissivematerials. See e.g. U.S. Pat. No. 6,590,707 (Weber). An effective mediumof birefringent material may thus be or comprise a stack of ultrathinlayers e.g. of alternating polymer materials, for example, where theoptical thickness of each of the layers is less than ¼, and preferablyless than ⅛, of a wave thick (e.g., less than 150, or 100, or 50 nmthick). Such media may in general be used in the disclosed embodiments.

EXAMPLES Example 1

A spatially tailorable optical film that reflected red light, referredto here as Film 1, was formed by: co-extruding about 300 alternatinglayers of two polymeric materials, one of which contained an infra-redabsorbing dye of suitable concentration; casting the extrudate into aquenched web; and stretching this cast web biaxially to form anoptically reflecting multilayer optical film that was spatiallytailorable.

More specifically with regard to Film 1, the two polymeric materialsincluded a high refractive index material and a low refractive indexmaterial. The high index material was a copolymer of polyethylenenaphthalate (PEN), and comprised 90 mol % naphthalene dicarboxylate and10 mol % terephthalate as carboxylates as described in Example 1 of U.S.Pat. No. 6,352,761 (Hebrink et al.), this copolymer containing both PENand polyethylene terephthalate (PET) sub-units, and referred to hereinas 90/10 coPEN. The lower index material was another copolymer of PEN(i.e., another coPEN) as described in Example 10 of U.S. Pat. No.6,352,761 (Hebrink et al.), this lower index material referred to hereinas 55/45 HD coPEN. A masterbatch comprising one wt. % infra-red (IR)absorbing dye (obtained under the trade designation “EPOLITE 4121” fromEpolin, Newark, N.J.), was formed by extrusion compounding the dyepowder into the 55/45 HD coPEN polymer. The masterbatch was furthermoreintroduced into the 55/45 HD coPEN resin feed stream for theco-extrusion process in the weight proportion of 1:17 to the purecopolymer. A feedblock separated the low index 55/45 HD coPEN into about150 layers, which were co-extruded in alternating fashion with about 150layers of the high index 90/10 coPEN material. The weight proportion ofthe material in the low index layers to the material in the high indexlayers was about 9:10. The outer layers of the coextruded film wereprotective boundary layers (PBLs) comprising the high index 90/10 coPENmaterial. The 300 alternating high and low index layers formed theso-called optical packet in the finished Film 1. A final co-extrudedpair of skin layers, comprising polyester material (obtained under thetrade designation “Eastman Copolyester SA-115B” from Eastman ChemicalCompany, Kingsport, Tenn.), was co-extruded in a total weight proportionof about 1:4 to the optical packet. The extruded web was quenched andthen furthermore heated above the glass transition temperature of thehigh index 90/10 coPEN material and stretched over rollers, in a lengthorienter, to a draw ratio of about 3.7, and then furthermore heated toabout 130° C. and stretched transversely to a draw ratio of about 4, ina tenter. The film was furthermore heatset after stretching to about215° C. The resulting Film 1 was about 35 micrometers thick with areflection band spanning from about 580 nm to about 680 nm, i.e.,reflecting red light. The transmission through a central portion of thisreflection band was about 2%.

A lenticular film (obtained under the trade designation “Dura-GO” fromTekra Corporation, New Berlin, Wis.) characterized by a thickness ofabout 0.46 millimeters (0.018 inches) and a cylindrical feature spacingof about 3 lines per millimeter (75 lines per inch), was applied to thered-reflecting Film 1 using an optically clear adhesive (obtained underthe trade designation “3M™ Optically Clear Adhesives”, type 8141, from3M Company, St. Paul, Minn.). The lenticular film was used as a mask todirect radiant energy to pattern the Film 1. The radiant energy wasprovided by a laser diode that emitted laser light at 808 nanometers.The beam emitted by the laser diode was scanned across the lenticularside of the optical construction in a scanning pattern perpendicular tothe cylindrical features of the lenticular film. If the average power ofthe beam was set too low, or if the scan speed was set too high, nosignificant change in the reflection characteristics of Film 1 wasobserved, demonstrating a threshold condition for processing. Severalcombinations of beam power and scan speed were found to successfullyprocess the Film 1, i.e., several combinations of these parametersproduced a significant change in the reflection characteristics of theFilm 1. In a first case, the average power of the beam was adjusted toabout 831 milliwatts, and the scan speed was about 60 millimeters/sec.The scan pattern comprised uni-directional line scans with a lineseparation of about 30 micrometers, these line scans covering a 119mm×119 mm processed region of the optical construction. In a secondcase, the average power of the beam was adjusted to about 362 milliwattsand the scan speed was about 15 millimeters/sec. The scan pattern inthis second case was again a series of uni-directional line scans, butin this case a smaller processed region, about 25 mm×25 mm in size, wastreated.

The processed optical construction was viewed under an opticalmicroscope. Viewing from the lenticular side under transmission,unprocessed regions of the optical construction appeared approximatelycyan in color, which is characteristic of a red reflector, while the twoprocessed regions described above appeared more white in color. Viewingfrom the lenticular side of the optical construction under reflection,the unprocessed regions appeared red in color, while the two processedregions were dark in appearance. The dark appearance is consistent witha reduction or elimination of reflectivity of the Film 1 in theprocessed regions. Viewing from the Film 1 side of the opticalconstruction under reflection, in the two processed regions, alternatingred stripes and dark (non-reflecting or reduced reflectivity) stripeswere observed, each of these groups of stripes having nominally the samespacing as the cylindrical features of the lenticular film. The darkstripes corresponded to focal areas under the cylindrical features ofthe lenticular film, and the red stripes corresponded to areas betweenthese focal areas. The focusing properties of the lenticular film wasthus observed to operate as a mask to divide each of the processedregions into first striped portions, whose reflectivity was about thesame as the original reflectivity of the unprocessed Film 1, and secondstriped portions of substantially reduced reflectivity, the first andsecond striped portions being self-registered with respect to thelight-focusing cylindrical features of the lenticular film.

The foregoing Example 1 could be repeated, except that the lenticularfilm could be removed from the optical construction after laserprocessing to isolate the patterned Film 1 from the lenticular film. Aremoveable adhesive between the Film 1 and the lenticular film could beused for this purpose. Alternatively, a structured surface mask such asthe lenticular film could be temporarily held in place over a STOF filmsuch as the Film 1 without adhesive, e.g. by a tensioning system withrollers, during laser processing, and then after laser processing thestructured surface mask could be removed.

Example 2

Two STOF films, Film 2 and Film 3, were made as described below. Film 2substantially reflected normally incident light at about 800 nanometers,and was used as a STOF mask for Film 3. Film 3 substantially reflectednormally incident yellow and red light, in a reflection band from about550 nm to 700 nm. Film 2 was formed by co-extruding substantially thesame high index and low index materials described in Example 1, i.e.,90/10 coPEN and 55/45 HD coPEN. A masterbatch comprising one wt. %infra-red (IR) absorbing dye (obtained under the trade designation“Amaplast IR-1050” from ColorChem, Atlanta, Ga.) was formed by extrusioncompounding the dye powder into the 55/45 HD coPEN polymer. Themasterbatch was furthermore introduced into the 55/45 HD coPEN resinfeed stream for the co-extrusion process in the weight proportion of1:17 to the pure copolymer. A feedblock was used to separate the lowindex 55/45 HD coPEN into about 150 layers, which were co-extruded inalternating fashion with about 150 layers of the high index 90/10 coPENmaterial. The weight proportion of the material in the low index layersto the material in the high index layers was about 9:10. The outerlayers of the coextruded film were protective boundary layers (PBLs)comprising the high index 90/10 coPEN material. The 300 alternating highand low index layers formed the so-called optical packet in the finishedFilm 2. A final co-extruded pair of skin layers, comprising polyestermaterial (obtained under the trade designation “Eastman CopolyesterSA-115B” from Eastman Chemical Company, Kingsport Tenn.), wasco-extruded in a total weight proportion of about 1:4 to the opticalpacket. The extruded web was quenched and then furthermore heated abovethe glass transition temperature of the high index 90/10 coPEN materialand stretched over rollers, in a length orienter, to a draw ratio ofabout 3.7, and then furthermore heated to about 130° C. and stretchedtransversely to a draw ratio of about 4, in a tenter. The film wasfurthermore heatset after stretching to about 215° C. The resulting Film2 was about 45 micrometers thick with a reflection band spanning fromabout 750 nm to about 850 nm. The transmission through a central portionof this reflection band was about 2%.

Film 3 was formed by co-extruding the high index 90/10 coPEN materialused in Films 1 and 2 and a different low index polymer material. Thelow index polymer of this Film 3 was a copolymer of polyethyleneterephthalate (i.e., a coPET) with 5 mol % carboxylate sub-unit moietysubstitution resulting from the use of sulfoisophthalic acid or itsesters, and 27% diol sub-unit moiety substitution resulting from the useof neopentyl glycol, described as polyester K in U.S. Pat. App. Pub. No.US 2007/0298271 (Liu et al.). About 0.55 wt. % of infra-red (IR)absorbing dye (obtained under the trade designation “Epolite 4121” fromEpolin, Newark, N.J.) was added to the high index 90/10 coPEN material,and this was co-extruded using a feedblock with the low index polymerinto about 550 alternating material layers. The outer layers of thecoextruded film were protective boundary layers (PBLs) comprising thehigh index 90/10 coPEN material. The 550 alternating high and low indexlayers formed the so-called optical packet in the finished Film 3. Afinal co-extruded pair of skin layers, comprising the 90/10 coPENmaterial without the absorbing dye, were also coextruded. The weightratios of the feedstreams for the high index material in the skinlayers, the high index material in the optical packet, and the low indexmaterial in the optical packet were about 6:11:16. The co-extruded layerstack was cast through a die, and formed into a cast web byelectrostatically pinning and quenching onto a chill roll. The cast webwas about 700 micrometers thick. The cast web was quenched and thenfurther heated above the glass transition temperature of the high index90/10 coPEN material and stretched over rollers, in a length orienter,to a draw ratio of about 3.7. Next, the film was heated to about 130° C.and stretched transversely to a draw ratio of about 3.5 in a tenter. Thefilm was furthermore heatset after stretching to about 235° C. Theresulting Film 3 was about 55 micrometers thick and had a reflectionband spanning from about 750 nm to about 850 nm. The transmissionthrough a central portion of the band was less than 1%. When viewedunder conditions favoring transmitted light from a white background, theFilm 3 exhibited a vibrant blue color. When viewed under conditionsfavoring reflected light, the Film 3 exhibited a vibrant gold color.

STOF Films 2 and 3 were then combined into a laminated construction thatincluded the Film 2, the Film 3, and a diffuse white polycarbonatesecurity film. The white polycarbonate film was about 150 micrometersthick, and was obtained under the trade designation “3M™ PC SecurityFilms” from 3M Company, St. Paul, Minn. These three films were adheredin the laminate with intervening layers of optically clear adhesive(obtained under the trade designation “3M™ Optically Clear Adhesives”,type 8141, from 3M Company, St. Paul, Minn.). When viewed underconditions favoring transmitted light from the white backing film, thelaminate exhibited a substantially blue color. When viewed underconditions favoring reflected light, the laminate exhibited asubstantially gold color.

The laminate was first treated using a first laser tuned to a wavelengthof 1064 nm. The first laser was a pulsed laser, and was set to a 375 kHzpulse frequency, 12 nanosecond pulse duration, delivered power of 3watts, and a beam width at the laminate of about 50 micrometers. A 2mm×2 mm region of the laminate was scanned with this laser beam with aline separation of about 50 micrometers, and a variety of linear scanrates including 250, 300, 350, 400 and 450 mm/sec. Using the scan rateof 250 mm/sec., the reflectivity of Film 2 throughout the treated regionwas observed to decrease significantly. The treatment with the firstlaser thus patterned the STOF Film 2 so that its reflectivity at about800 nm outside of the treated region was about the same as that of theuntreated Film 2, and its reflectivity at about 800 nm inside thetreated region was substantially reduced.

This patterned Film 2 was then used as a mask in the treatment of theSTOF Film 3 in the laminate, using a second laser. The second laser wasa diode laser tuned to 808 nm. The beam provided by the diode laser wascontinuous rather than pulsed, and it had a delivered power of about 3watts and a beam width at the laminate of about 50 micrometers. The beamprovided by this second laser was directed at the laminate and scannedover a second region that was wider than the first region treated by thefirst laser (at the 250 mm/sec. scan rate). The second region coverednot only the first region of the laminate but also other regions of thelaminate that were untreated by the first laser. The beam of the secondlaser was scanned over the second region at a linear rate of 64 mm/sec.with a line separation of 100 micrometers. After treating the laminatewith the second laser in the second region, the reflective properties ofFilm 3 were seen to change in the area of overlap between the first andsecond regions. When viewed under conditions favoring transmitted lightfrom the white backing film, the laminate exhibited a substantiallywhite color in the area of overlap between the first and second treatedregions, indicative of the white polycarbonate backing film, which wasnow visible through the mask portion (the first region) of Film 2 andthrough the treated portion of Film 3 (in the area of overlap betweenthe first and second treated regions). In other regions of the laminate,such as the portion of the second treated region that did not overlapwith the first treated region, the laminate maintained its initialcolored appearance, resulting from the unchanged reflectivecharacteristics of Film 3. Thus, in these other regions, the laminateexhibited a substantially blue color when viewed under conditionsfavoring transmitted light from the white backing film, and the laminateexhibited a substantially gold color when viewed under conditionsfavoring reflected light. The treatment of the laminate with the secondlaser thus accomplished the patterning of the STOF Film 3 using as amask a STOF film (Film 2) that had been first spatially patterned by thefirst laser.

The foregoing Example 2 could be repeated, except that the patternedFilm 2 could be removed from the laminate construction to isolate thepatterned Film 3 from the mask provided by patterned Film 2. A removableadhesive between the mask and the rest of the laminate could be used forthis purpose. Alternatively, a STOF mask such as the patterned Film 2could be temporarily held in place over a second STOF film such as Film3 without adhesive, e.g. by a tensioning system with rollers, duringprocessing with the second laser, and then after such processing theSTOF mask could be removed.

Unless otherwise indicated, all numbers expressing quantities,measurement of properties, and so forth used in the specification andclaims are to be understood as being modified by the term “about”.Accordingly, unless indicated to the contrary, the numerical parametersset forth in the specification and claims are approximations that canvary depending on the desired properties sought to be obtained by thoseskilled in the art utilizing the teachings of the present application.Not as an attempt to limit the application of the doctrine ofequivalents to the scope of the claims, each numerical parameter shouldat least be construed in light of the number of reported significantdigits and by applying ordinary rounding techniques. Notwithstandingthat the numerical ranges and parameters setting forth the broad scopeof the invention are approximations, to the extent any numerical valuesare set forth in specific examples described herein, they are reportedas precisely as reasonably possible. Any numerical value, however, maywell contain errors associated with testing or measurement limitations.

Various modifications and alterations of this invention will be apparentto those skilled in the art without departing from the spirit and scopeof this invention, and it should be understood that this invention isnot limited to the illustrative embodiments set forth herein. Forexample, the reader should assume that features of one disclosedembodiment can also be applied to all other disclosed embodiments unlessotherwise indicated. It should also be understood that all U.S. patents,patent application publications, and other patent and non-patentdocuments referred to herein are incorporated by reference, to theextent they do not contradict the foregoing disclosure.

The invention claimed is:
 1. A composite article comprising: a film maskincluding a first group of layers arranged to extend continuously from afirst zone to a second zone and selectively reflect light byconstructive or destructive interference to provide a first reflectivecharacteristic in the first zone and a second reflective characteristicin the second zone, the film mask having a first absorbingcharacteristic suitable to, upon exposure to a first radiant beam,absorptively heat a portion of the film mask by an amount of sufficientto change the first reflective characteristic to the second reflectivecharacteristic, wherein the change from the first reflectivecharacteristic to the second reflective characteristic is substantiallyattributable to a change in birefringence of at least some of the firstgroup of layers of the film mask; and a patternable film beneath thefilm mask and connected to the film mask in a layered arrangement, thepatternable film having a first detectable characteristic at a selectedportion that changes to a different second detectable characteristicupon exposure to a second radiant beam through the film mask, whereinthe film mask is capable of patterning the second radiant beamtherethrough to change the first detectable characteristic to the seconddetectable characteristic at the selected portion of the patternablefilm.
 2. The composite article of claim 1, wherein the patternable filmcomprises a material that is excited at the second radiant beam.
 3. Thecomposite article of claim 2, wherein the material of the patternablefilm comprises a fluorescent dye that emits at visible wavelengths uponexposing to the second radiant beam.
 4. The composite article of claim1, wherein the first reflective characteristic reflects the secondradiant beam more than the second reflective characteristic.
 5. Thecomposite article of claim 1, wherein the selected portion of thepatternable film corresponds to the second zone of the film mask.
 6. Thecomposite article of claim 1, wherein the first reflectivecharacteristic reflects the second radiant beam less than the secondreflective characteristic.
 7. The composite article of claim 1, whereinthe selected portion of the patternable film corresponds to portions ofthe film mask other than the second zone.
 8. The composite article ofclaim 1, wherein the first and second radiant beams comprise differentfirst and second optical wavelengths, respectively.
 9. The compositearticle of claim 8, wherein the first optical wavelength is an infraredoptical wavelength, and the second optical wavelength is less than 700nm.
 10. The composite article of claim 1, wherein the first or secondreflective characteristic has a reflectivity for the second radiant beamof at least 90%.
 11. The composite article of claim 1, wherein thearticle is a security construction including an identification card, apassport, or a driver's license.
 12. The composite article of claim 1,wherein the article is a security document, and the film mask comprisesa personalizable feature in reference to a document holder.
 13. Thecomposite article of claim 1, wherein the film mask and the patternablefilm are layered to provide combined security features.
 14. A method ofmaking a composite article, comprising: providing a first film having afirst reflective characteristic, the first film also having a firstabsorption characteristic suitable to, upon exposure to a first radiantbeam, absorptively heat a portion of the first film by an amountsufficient to change the first reflective characteristic to a secondreflective characteristic by a change in birefringence, and the firstfilm comprising a first group of layers arranged to selectively reflectlight by constructive or destructive interference to provide the firstreflective characteristic, wherein the change from the first reflectivecharacteristic to the second reflective characteristic is substantiallyattributable to a change in birefringence of at least some of the firstgroup of layers of the first film; providing a patternable film having afirst detectable characteristic that changes to a different seconddetectable characteristic upon exposure to a second radiant beam,wherein the patternable film comprises a material that is excited at thesecond radiant beam; directing the first radiant beam preferentially ata second zone rather than a first zone of the first film to change thefirst reflective characteristic to the second reflective characteristicin the second zone by a change in birefringence so as to convert thefirst film to a film mask; and arranging the film mask and thepatternable film in a layered structure, using the film mask to patternthe second radiant beam, and directing the patterned second radiant beamat the patternable film to change the first detectable characteristic tothe second detectable characteristic at a selected portion of thepatternable film.
 15. The method of claim 14, wherein directing thefirst radiant beam preferentially at the second zone of the first filmcomprises scanning the first radiant beam over portions of the firstfilm that define the second zone.
 16. The method of claim 14, whereinthe selected portion of the patternable film corresponds to the first orsecond zone of the first film.
 17. The method of claim 14, wherein thefirst and second radiant beams comprise different first and secondoptical wavelengths, respectively.
 18. The method of claim 17, whereinthe first optical wavelength is an infrared optical wavelength, and thesecond optical wavelength is less than 700 nm.
 19. The method of claim14, wherein the material of the patternable film comprises a fluorescentdye that emits at visible wavelengths upon exposing to the secondradiant beam, and directing the patterned second radiant beam at thepatternable film forms a pattern of visible fluorescent light.